Abstract: Reasonable determination of the cyclic strength of marine clay is critical for ensuring the stability of marine structures throughout their service life. A series of constant-volume cyclic direct simple shear tests were performed on the undisturbed saturated marine clay in the Yangtze estuary with different plasticity indexes (IP) under different cyclic stress ratios (CSRs). The cyclic responses of the marine clay specimens were presented. The cyclic failure criterion was investigated by employing the energy method. The results indicate that there is a threshold cyclic stress ratio (CSRth) in the undisturbed marine clay. When CSR is smaller than CSRth, the energy dissipation per cycle (Wi) develops linearly only within a limited range that does not contribute to the cyclic failure of marine clay. However, when CSR exceeds CSRth, the development curve of Wi with the number of cycles (N) shows an inflection point, due to the serious damage of the soil structures. This point serves as a critical point for cyclic failure, determining the number of cycles to failure (Nf) and the double-amplitude shear strain to failure (γDA,f). The CSRth of the marine clay in the Yangtze River estuary exhibits a power function relationship with the increasing IP. Additionally, with the increasing CSR and IP, both Wi and γDA,f tend to increase, while the Nf gradually decreases. Furthermore, the data points of γDA,f/IP1.5 ~ CSR - CSRth for all tests are distributed in a narrow band, and a virtually positive linear relationship exists between the γDA,f/IP1.5and CSR - CSRth. Finally, a γDA,f evaluation method applicable to marine clay in different seas is presented for practical geotechnical engineering.