The emergence of ultrafast laser spectroscopy technology has greatly promoted the deep understanding of the microscopic processes of ultrashort time scale (such as femtosecond) in many research fields. The principle and techniques of femtosecond time-resolved transient absorption spectra are introduced in detail. Combined with our research results, the application of this method in the study of the microscopic mechanism of ultrafast relaxations and interactions in the excited states of condensed molecularsystems is demonstrated, especially for relaxation of excited states, evolution pathway and coherence of wavepacket, energy transfer, proton/charge transfer, molecular structural dynamics and so on. It is indicated that this method will play an importantrole in other related fields such as physics, chemistry, materials, biology, environment and other interdisciplinary fields. Finally, the potential developments and further research trends of the ultrafast spectroscopy are prospected.