The carrier relaxation process and the time evolution process of the conductivity in the terahertz band of the ZnSe nanocrystal are investigated by op tical-pump terahertz-probe spectroscopy at room temperature.Monitoring the chan ges of the THz probe pulse,the transient photoconductive properties of ZnSe nan ocrystals induced by photoexcited carriers were systematically studied,and the transient conductivity was fitted with the Drude-Smith model.With the laser pu l se excitation at 400nm and the pump power is 10mW and 16mW,the negative tran smission of terahertz pulse shows an ultrafast rising process followed by a reco very process.The recovery process obviously consists of two parts:fast and slo w,and can be well fitted with a biexponential decay curve.The fast decay compo nent with time constant of 5ps is mainly controlled by the backscattering of ph otocarriers at the interface defects of ZnSe nanocrystal,and the slow decay com ponent with time constant greater than 1ns is mainly caused by the recombination of carriers from the conduction band to the valence band.The evolution of cond uctivity with time demonstrates that this kind of nanostructure is a good candid ate for fabricating ultrafast terahertz switching.