With the help of gain guiding effect, the quality of the beam which emitted from master oscillator power amplifier (MOPA) laser system can be improved without any compensations. Considering thermal effect, gain guiding effect and gain saturation effect comprehensively, a novel model for laser amplifier in MOPA system is developed and numerically caculated. The simulation result approximately agrees with the recently reported experimental counterpart. Moreover, the simulation result reveals that fill factor and pump power are two key aspects influencing the final beam quality of the MOPA system. Besides, the thermal effect and gain guiding effect are the underlying reasons. This analysis is meaningful for improving beam quality of laser emitted from amplifier.