The nonlinear dynamic characteristics of distributed feedback (DFB) semiconductor lasers subjected to external optical injection have been investigated experimentally, and the output dynamical characteristics are mapped as a function of the power of optical injection and the frequency detuning between the injection frequency and free-running frequency of the unperturbed DFB semiconductor laser. Experimental results show that DFB semiconductor lasers are very sensitive to external perturbation and can exhibit periodic, chaos and other complex nonlinear dynamical characteristics when subjected to different external optical injection. There exist three regions of complex dynamics in the map. The complex dynamics is always related to the multi-periodic dynamics. When the laser exhibits complex dynamic, its spectrum is continuous spectrum, and there are several peaks in the spectrum. The magnitude of frequency that the peaks correspond to respectively has nothing with the power and the frequency of injection field.