Terahertz (THz) radiation is generally defined as the region of the electromagnetic spectrum in the range of 0.1 to 10 THz, between the millimeter and infrared frequencies. THz radiation is important from both scientific and application point of view. THz science and technology has been an active research area for a wide variety of applications: such as spectroscopy, imaging and sensing, biology and medical sciences, and security evaluation. The development of efficient, ultra-broadband, and low-cost THz photonic devices requires new materials and mechanisms, which is the key challenge for the field of THz science and technology. The discovery of THz electromagnetic pulse emission from ultrafast demagnetization by femtosecond laser pulses gave insight into the microscopic interactions that connect the ultrafast spintronics and the THz photonics.Based on our experimental observations, this paper reviews the recent developments and applications, the current understanding of the physical processes, and the perspectives of ultrafast spin-based THz photonics.Firstly, ultrashort THz pulses have been demonstrated as a promising tool to investigate the ultrafast spintronics. We review the fundamental physical processes and properties including THz-driven spin waves, THz spin transport probing, and ultrafast THz magnetometry. 1) The THz pulses are used to excite and control the antiferromagnetic spin waves in rare-earth orthoferrites with the THz time-domain spectroscopy. In addition, we observe the magnon-polariton, magnon-spin coupling, and magnon-magnon coupling in the condensed matter systems. 2) We demonstrate the magnetic modulation of THz waves, along with heat- and contact-free giant magnetoresistance, tunneling magnetoresistance and anisotropic magnetoresistance readout using ultrafast THz signals. We directly determine the spin-dependent densities and momentum scattering times of conduction electrons. The various magnetic configurations between the parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the magnetic tunnel junctions have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. 3) We demonstrate a method of ultrafast THz magnetometry, which indicates the sub-picosecond demagnetization dynamics in a laser-excited iron film. The measurements reveal the contributions originating from magnetization quenching and acoustically-driven modulation of the exchange interaction. In addition, the ultrafast photoinduced spin transport can be extracted from the THz emission signals. We observe the transition of laser-induced THz spin currents from torque-mediated to conduction-electron-mediated transport in ferromagnetic/non-magnetic heterostructures.Secondly, by exploring the ultrafast THz spintronic effects, new applications in THz photonic devices emerge, including spintronic THz emitters, THz modulators and THz detectors. 1) The ferromagnetic/non-magnetic heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency THz emission. The ultrafast spin-charge conversion based on the Inverse Spin Hall Effect (ISHE) is used to generate broadband THz radiation. We summarize the efforts that have been made to improve the performance of spintronics-based THz emitters. Up to date, the efficiency of spintronics-based THz emission has been enhanced to reach the same level of millimeter-thick ZnTe crystal. 2) The combined spintronic and photonic heterostructures are exploited to realize active modulation of THz radiation. In addition, it is demonstrated that the THz radiation can be mediated coherently through the charge current induced by the ISHE and the built-in transient current quasi-simultaneously created within the patterned heterostructures. 3) Using the ISHE, an antiferromagnet/heavy metal bilayer is theoretically promising for the realization of a resonant, compact, and tunable THz detector. In addition, a coherent and phase-locked coupling between a single-cycle THz transient and the magnetization of cobalt films suggests new opportunities for THz pulse detection.Finally, a brief summary and outlook are given. Looking to the future, we introduce the applications of ultrafast spin-based THz photonics, such as ultra-broadband measurements, magnetic structure detection and imaging, and THz near-field microscopy. In addition, topological materials bear a large potential for efficient spin-to-charge conversion due to the inherent spin-momentum locking. The topological insulator/ferromagnetic heterostructures are expected to present a high-performance THz radiation. In addition, the topological spintronic THz emitter will show a potential to generate arbitrary THz waveforms. One can anticipate that the research scope of ultrafast spin-based THz photonics will successfully be used to understand the fundamental physics in new materials and give rise to high-efficient THz photonic devices and spectroscopy applications. We hope that our work will stimulate more fundamental and technological developments in this new research field.