With the development of semiconductor technology, semiconductor laser devices and semiconductor laser pump solid-state laser devices have been widely applied in z-scan experiments. However, the feedback light-induced output instability of semiconductor laser devices can negatively affect the accurate testing of the nonlinear index. In this work, the influence of feedback light on z-scan measurement is analyzed. Then the calculated formula of feedback light-induced false nonlinear z-scan curves is theoretically derived and experimentally verified. Two methods are proposed to reduce or eliminate the feedback light-induced false nonlinear effect. One is the addition of an attenuator to the z-scan optical path, and the other is the addition of an opto-isolator unit to the z-scan setup. The experimental and theoretical results indicate that the feedback light-induced false nonlinear effect is markedly reduced and can even be ignored if appropriate parameters are chosen. Thus, theoretical and experimental methods of eliminating the negative effect of feedback light on z-scan measurement are useful for accurately obtaining the nonlinear index of a sample.