Abstract: A new principle for spontaneous-pulse-mode-locked laser is first presented,in this paper.The principle mainly is that a optical element is inserted into a single straight cavity to construct a nonlinear cavity to realize the spontaneous mode-locking operation.The physical model and mathematical equations of the laser are established. The results of numerical calculation have been analyzed.The selt-starting APM condition,the optimum operating condition and the relationship between design parameters and ratio of pulse-width-shortening are analyzed.The calculation results can be used in various laser system.