Abstract: The cone penetration test, which is more and more important in geotechnical investigation, has been widely used in the offshore engineering at home and abroad as the deep sea is gradually developed. The history of the cone penetration test is summarized, especially in the offshore engineering. And the full-flow penetrometer is introduced, which is fairly suitable for geotechnical investigation in deep see. Compared with the conventional techniques, the full-flow penetrometer can ignore the soil weight and pore pressure in the test. A large projection area of the probe will get more accurate results. A strict theoretical solution is worked out about the relation between the penetration resistance and the undrained shear strength of soft clay. The cyclic penetration test can evaluate the remoulded shear strength and the sensitivity of soil. A numerical analysis by appling the CEL method is conducted for the penetration process of the full-flow penetrometer. The simulated results show that the flow mode of soil is quite different with that of the theoretical assumption as the T-bar penetrates but it is similar when the Ball penetrates. The bearing capacity factor calculated by the numerical results is smaller than that suggested by Randolph, both T-bar and Ball.