The quasi-three energy levels laser rate equation model of Tm:YAP slab pumped by two fiber coupled diode lasers is developed. Based on the solution of the equation, the influence on Tm:YAP laser output power is simulated, including the working temperature distribution of the crystal, up-conversion effect, the appropriate range of the spot radius and the transmission efficiency of output lens. It provides the necessary numerical analysis basis of designing the high power Tm:YAP slab laser.