A numerical model for propagation of optical pulse in fiber is established based on nonlinear Schrdinger equation (NLSE), and the formation mechanisms of self-similar pulse in the oscillator are studied. Self-similar pulse evolution in laser is available under reasonable parameters. The pulse, with strict linear chirp, can be compressed to 90 fs, and its peak power reaches to 26 kW. Pulse width is broadened or compressed along with the different positions of the oscillator. This is because of the interaction of self-phase modulation and group velocity dispersion in fiber. With the increase of net dispersion of the cavity, the output pulse energy increases gradually, while the pulse breathing ratio decreases gradually. Whereas, with the increase of pumping power, both pulse energy and pulse breathing ratio increase. This self-similar pulse can be compressed to near transform-limited with ultra-high peak power.