In this paper, a new principle of in-situ testing penetrating resistance of marine sediment is given and analyzed simply. The principle is to transform the measurement of marine sediment's penetrating resistance strength into the change of the oil pressure in the sealed vat. During the transformation, the penetrating resistance strength is magnified in some times, and is transmitted to the sensor in the form of the change of the oil pressure. By processing the data from the sensor, the original value of the penetrating resistance strength of marine sediment can be obtained.