Based on the master oscillator power amplifier(MOPA) system including a single mode erbium-doped fiber laser used as the master oscillator,and a large-mode-area Erbium/Ytterbium double-clad fiber used as the amplifier gain medium,the polarization states of the seeding laser and the amplified laser are experimentally investigated.The results show that the polarization degree of the output laser of the amplifier is very low without any polarization control device;after inserting a polarizer in the loop of the signal source,the degree of polarization is improved evidently and the stability of the polarization state becomes better,both for continuous wave operation and for mode-locked operations.The mode-locked operation has a little lower polarization degree and a better stability than the continuous wave operation of the amplifier.When a polarizer introduced in the loop of the signal,the instability of the polarization state of the output of amplifier is mainly caused by the polarization parameter azimuth.As a result,the combination use of a polarizer and a polarization controller is supposed to improve the polarization degree and the stability of the output of the amplifier,furthermore,polarization-maintaining gain fiber is recommended to be used to avoid the random perturbation of the environment.The work offers an important experimental basis for the control of the polarization state in the laser beam coherent combination.