Abstract: In order to study the mechanical properties of coral seawater and sea sand concrete (CSSC) under the combined action of compression and shear, 21 coral seawater and sea sand concrete specimens were designed and manufactured with the compressive stress ratio as the variable parameter. The failure modes of the specimens under the combined action of compression and shear were observed. The influence of compressive stress ratio on the shear strength of CSSC was analyzed in depth, and the shear strength calculation formula and failure criterion were proposed. The results show that with the increase of compressive stress ratio k, the shear strength of CSSC increases approximately in a power function, and the brittle failure characteristics of the specimen gradually weaken. When k increases from 0 to 0.5 with an increment of 0.1, the shear strength increases by 1.53, 2.81, 3.60, 4.32 and 4.67 times, respectively. The shear strength of coral seawater and sea sand concrete is mainly composed of cohesive strength, aggregate interlocking strength and interfacial friction strength, which account for 10%~22%,19%~30% and 50%~69% of the shear strength, respectively. With the increase of the compressive stress ratio, the cohesive strength first increases and then decreases, the aggregate occlusion strength generally shows an upward trend, and the interface friction strength increases approximately linearly. According to the experimental data, the calculation formula of CSSC shear strength is proposed, and the calculated value is in good agreement with the experimental value. Different failure criteria are used to analyze the test data, and it is found that the failure criteria based on the principal stress space are in good agreement with the test results.