The influencing factors are experimentally investigated on the output linear polarization deterioration to elliptical polarization in the semiconductor optical amplifier’s (SOA) nonlinear polarization rotation (NPR). In a stable laboratory environment condition, with changing separately the initial polarization state of optical signal, SOA initial bias current and input optical power. According to the problem of output line polarization deterioration, the control variation method is used. Experimental results show that the initial polarization state of the optical signal has little influence on the linear polarization deterioration, the SOA initial bias current and input optical power play the main roles. The smaller the initial bias current or the greater the input optical power is, the more the output linear polarization deteriorates. The study would be useful for the design of the application system such as optical switch, all-optical signal processing system based on SOA’s NPR, as well as providing solid evidence for the research on the mechanism of the output linear polarization deterioration.