A new optimal guidance law based on nonsingular terminal sliding mode was proposed for the deficiency existing in the traditional optimal guidance law based on linear sliding mode such as poor robustness, asymptotic convergence of line-of-sight angle error and its rate. Singularity in the traditional terminal sliding mode caused by parameter's improper chose is avoided by the improved nonlinear sliding mode and the finite-time convergence of error vector is achieved. Controlled by guidance law, the rate of line-of-sight angle can converge quickly, ensuring higher guidance precision. Simulation results show that the guidance law proposed is effective and robust.