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This study conducts a series of undrained cyclic shear tests on saturated coral sand with different initial relative densities (Dr) subjected to a 90° jump rotation of the principal stress with various initial orientations. Considering liquefiable coral sand as a fluid, an important finding is that the variation in the apparent viscosity with the number of cycles is significantly affected by Dr, the effective mean principal stress (p0'), cyclic stress ratio, cyclic loading path, and cyclic loading frequency. The average flow coefficient (κ) is introduced to describe the fluidity of saturated coral sand under cyclic loading by considering the variation in the shape of the stress-strain rate curve. A positive exponential correlation is observed between κ and the excess pore water pressure ratio (ru) under the given cyclic loading modes. Another significant finding is that the solid-liquid phase change time is the turning point of the apparent viscosity and the average flow coefficient gradients with ru. The inversion point’s excess pore water pressure ratio is approximately 0.9, defined as the excess pore water pressure ratio during the solid-liquid phase transformation (ruth). The ruth value is not affected by cyclic loading mode, p0', and Dr.