In this paper, the ultrafast optical response of chemical vapor deposition(CVD) grown ReS₂ thin films is investigated by means of ultrafast time-resolved spectroscopy. The optical pump probe study shows that the ReS₂ thin films have ultrafast carrier thermalization processes and sub-nanosecond scale compounding processes. Optical pumping terahertz emission tests show that the ReS₂ films are capable of producing terahertz radiation with a spectral width of 2.5 THz under femtosecond laser pumping. The polarity of the terahertz radiation is reversed with the change of the pump light incidence angle. The analysis shows that the main physical mechanism of terahertz emission from femtosecond-pumped ReS₂ films is the surface field effect. The microscopic mechanism elucidated in this study has important reference value for the application of ReS₂ thin films in ultrafast and terahertz optoelectronic devices.