The transient photocurrent generated by femtosecond laser excitation on the surface of Dirac semimetal ZrTe₅ were measured through the reflected terahertz time-domain spectroscopy. Experimental results reveal several physical mechanisms of terahertz radiation generated from ZrTe₅. The results show that the polarization-independent photocurrent is the main component of the surface current， and the terahertz amplitude is related to polarization of the pump laser pulse， which indicates that part of the current is caused by nonlinear optical rectification effect. Under circularly polarized laser pumping， the terahertz amplitude changes four times periodically with the pump laser pulse， which confirms that circular photogalvanic effect of ZrTe₅. Besides， we have analyzed the terahertz time-domain electric field excited by ultrashort laser pulses. The ZrTe₅ inversion symmetry is broken under the excitation of the femtosecond laser pulse， which produces B_1u phonons， forming a transient Weyl point， and undergoing a phase transition from the Dirac state to the Weyl state. This is of great significance to the study of topological phase transitions and other topological states.