The classic phase diversity method in wavefront sensing, which utilizes defocusing as a means of phase diversity, necessitates precise knowledge of the defocus amount, a requirement often difficult to meet in numerous applications and thereby hindering its practical usage. Addressing the issue of potential defocus errors encountered in practical applications, conducts a simulation analysis on the effect of the defocus amount errors on the accuracy of the classic phase diversity method in wavefront sensing was conducted, as well as its effect on the error evaluation function. Consequently, a defocus amount search algorithm based on the error evaluation function is proposed to solve the problem of inaccuracies in defocus, enhancing usability in practice. This paper also verified and analyzed the feasibility and performance level of the method through extensive simulation and statistical analysis.