Terahertz pulse radiation can be generated by pumping semiconductor surfaces or semiconductor heterojunctions with ultrashort laser pulses. Based on the model of terahertz generation from metal-insulator-semiconductor heterostructure pumped by ultrashort laser pulses, the changes of carrier density and velocity in semiconductors are studied through numerical simulations and theoretical analysis. The influences and physical mechanisms of ultrashort laser pulse duration, carrier lifetime, and semiconductor thickness on the terahertz generation are analyzed as well. The results show that the increase of the laser pulse duration increases the amplitude of the terahertz pulse but decreases its central frequency and bandwidth. The increases of the carrier lifetime and thickness of the semiconductor have different influences on the central frequency and bandwidth of terahertz pulse. By analyzing the influence of different parameters on the terahertz generation, the pathways and parameters of optimizing the terahertz generation are obtained. The results of this paper provide a good theoretical reference for the related experiments.