In high power master oscillation power amplification (MOPA) laser, the amplified spontaneous emission (ASE) and induced self-excited oscillation have significant effects on output characteristics of lasers. Relationship between the self-excited oscillation and fiber end surface reflectivity is analyzed theoretically. Based on the self-designed MOPA fiber laser system, relationships between the self-excited oscillation and gain fiber length, pump power, signal power are investigated experimentally. According to the experimental results, the output characteristic curves of MOPA fiber laser are summarized. Results show that when the amplification stage ranges from 10 to 15, the proportion of self-excited oscillation light in output spectrum is the smallest. Experimental results provide a reference for the optimization of MOPA fiber amplifier and improvement of signal to noise ratio of output laser.