In this paper, a theoretical model of Tm3+ doped silica fiber laser(TDSFL) for 3H6—3H4 pumping mechanism is established based on the rate-equation theory, and the numerical simulation is performed by using the Runge-Kutta algorithm. The introcavity power distributions of the TDSFL are obtained for both pump and laser, and the effects of Tm3+ doped fiber length and Tm3+ concentration on laser performance are analyzed.