Terahertz communication has become one of the key candidate techniques for future 6G mobile communication to realize "communication with full spectra" because of its rich spectrum resources. Due to the unique advantages such as simple structure, easy integration, low cost and low power consumption, the incoherent photonics-assisted terahertz communication system based on envelope detection technique has been widely concerned and favored by the researchers. By summarizing the structure, characteristics and key techniques of incoherent photonics-assisted terahertz communication systems, current research status and outstanding achievements of the terahertz envelope detectors and the incoherent photonics-assisted terahertz communication systems at home and abroad in recent years are introduced. In addition, the incoherent photonics-assisted terahertz communication system based on amplitude modulation and In-phase Quadrature(IQ) modulation are discussed in detail, and the advantages and disadvantages of each system are also compared. Finally, the future development directions of 6G-oriented incoherent photonics-assisted terahertz communication systems and the crucial devices are expected.