In order to study the properties of material of phase-Ⅱ cofferdam cutoff wall in Three Gorges Project, a test of the material moulding under ground water is conducted to compare with that of ordinary moluding. The test result shows a notable difference of the mechanical index of the material between two cases. That is the compression strength of the material with underwater moulding is 40%-20% lower than that of ordinary moulding, and the initial elastic mouldus of the former is 40%-10% smaller than the latter, but the difference decreases gradually with ages. The research result provides a scientific basis to mend design parameters of the cutoff wall, as well as to check and accept the cutoff wall.