The experimental results observed in passively mode-locked Er-doped fiber laser with positive dispersion are analyzed theoretically. The static pulse characteristics are simulated numerically according to Master equation, and the conclusion is that stable passively mode-locked pulse with large chirp will be obtained in the cavity with positive dispersion. The evolvement process of pulse is simulated by coupled nonlinear Schrdinger equations, and the original small signal with large time-width evolves to stable mode-locked pulse after some cycles. The multiple-pulse operation observed in this laser is analyzed, and the results display that the multiple-pulse is caused by peak power limiting. The multiple-pulse operation evolvement is simulated, and the same results with experiments was obtained.