The perturbation approach is applied to solve the nonlinear Schrodinger equation, and its valid range has been determined by comparing with the results of the split-step Fourier method over a wide range of parameter values. With γ= 2㎞^-1mW^-1, the critical distance for the first order perturbation approach is estimated to be(equation omitted). The critical distance, Z_c, is defined as the distance at which the normalized square deviation compared to the split-step Fourier method reaches 10^-3. Including the second order perturbation will increase Z_c more than a factor of two, but the increased computation load makes the perturbation approach less attractive. In addition, it is shown mathematically that the perturbation approach is equivalent to the Volterra series approach, which can be used to design a nonlinear equalizer (or compensator). Finally, the perturbation approach is applied to obtain the sinusoidal response of the fiber, and its range of validity has been studied.