Transient processes of semiconductor laser were simulated. These processes are best described by the variations of injected electron density and emitted photon density. Rate equations were chosen to describe the transient processes. Using the described model of transient processes, the unknown parameters of DFB (distributed feedback) semiconductor laser were defined from the experimental characteristics: the coefficient of optical amplification α, the factor of spontaneous emission β, the electron and photon lifetime, and the form of injection current pulse. The parameter estimation technique, which allows to define laser parameter values simply, quickly, and fairly precisely, was suggested.Transient processes were simulated for several DFB lasers and the coincidence of calculation results with experimental ones for all lasers was sufficient. The usable physical model was improved. Transient processes of lasers were simulated again and more precise results were obtained. The mismatch of analysed laser parameters with experimental ones did not exceed the limit of 10%.