Abstract: For the purpose of choosing correct parameter in future experiments,numerical analysis of the transient-state of the Yb3+-doped double clad fiber laser used in the master-oscillator power amplifier(MOPA) system has been preformed.The pump power density distribution and reverse population density distribution,the effect of the fiber length and input light pulse energy on the output power have been analyzed.These results are benefit for the design of MOPA's parameters such as fiber length,pump power and master-oscillator's power in experiment.