An equivalent transform method for evaluating the effect of water column mismatch on geoacoustic inversion is presented. This quantitative method is derived from perturbative inverse formulations and gives a quantitative way to evaluate how the water column mismatch error transfers into the bottom inversion results. In this paper, this method is illustrated with two test cases, in which linear and nonlinear internal waves are considered as the cause of the water column mismatch, respectively. In the first case, the range-averaged geoacoustic inversion errors due to the water column mismatch are largely eliminated after a full cycle of the linear internal wave, and the error pattern repeats with the period of the internal wave. However, in the second case, the errors are accumulated but scaled down with increasing range. Additionally, ignoring the lower-frequency components of the signal removes its corresponding contribution from the overall errors, and reduces the bottom depth influenced by water column mismatch.