Particle in cell ;PIC simulation is a feasible approach to understand the interaction of ultra-intense ultra-short pulse lasers.The output data from the simulation are often nonstationary signals varying in temporally or spatially.So the calculation of time-frequency distribution of these signals is introduced,and the results show that this technique is effective for diagnosing the time-varying ;or space-dependent signals from the simulation of the interaction of ultra-intense ultra-short pulse lasers.As an example,the stimulated Raman scattering for linear chirped pump laser in near 1/4 critical density of inhomogeneous plasma is investigated and a clear picture of the physical process is obtained by time-frequency distribution technique.