Amplitude-variation-with-offset (AVO) inversion is a common approach to estimate elastic parameters. In conventional AVO inversion, various approximations of the Zoeppritz equation have been used. However, the weak elastic contrast assumption of those approximations leads to big errors when dealing with the seafloor situation in which the contrast of the elastic parameters across the interface is very strong. To extend the application of AVO to seafloor elastic parameters estimation as a linear problem, we developed a high-contrast approximation of the P-wave reflection coefficient to the Zoeppritz equation via a series truncation procedure. Model tests determined that the approximation we derived was more applicable than conventional approximations when considering the seafloor case. Any approximation limits AVO inversion to a small incidence-angle range. A two-step inversion method using the joint application of approximation and exact equation was evaluated to overcome the angle limit. Then we determined the influence of P-wave reflection coefficient accuracy on the inverted elastic parameters, and we developed a new technique to invert for seafloor elastic parameters directly by amplitude-versus-angle data without estimating reflection coefficient first. Finally, we applied the technique to estimate the seafloor parameters from AVO data of the South China Sea.