The calculation model for bottom reverberation was improved with the closed-form formulas based on Lambert′s law and the numerical computation method of curve fitting, in order to solve the problem of traditional bistatic bottom reverberation. Different forms of non-flat bottom were designed and their bistatic reverberation were researched. The influence of fitting steps on the computational accuracy of signal-to-reverberation-ratio was also discussed in the situation of periodic or aperiodic non-flat bottom. The simulation showed that the proposed method can quickly get the signal-to-reverberation-ratio of complex bottom terrain when the fitting step is appropriate. Compared with the classical elemental scattering model, the developed model can improve the computational efficiency greatly, while simultaneously keeping the accuracy.