The invention relates to a hydraulic supporting body for a direct tensile test device. A tensile device is formed by stacking cylindrical members which have a common central axis and comprises upper and lower end caps for fixing a sample, upper and lower chains connected with the two end caps respectively and upper and lower connectors connected with the other ends of the upper and lower chains respectively, wherein the lower connector is fixed on the base of a test machine. The supporting body consists of a set of hydraulic cylinder containing liquid and is divided into a supporting cylinder and a measuring cylinder, wherein the bottom parts of the two cylinders are communicated by a hollow conduit. In a test process, according to the difference between heights of the variable liquid level in the measuring cylinder, the value of the buoyancy provided by a hydraulic pressure can be calculated and removed from tensile data, and the system errors are reduced. And in the invention, the hydraulic pressure is used in place of the conventional spring, the measurement range of test data is enlarged, the operation is more convenient, the measurement accuracy is improved, and the test device is protected from being damaged in time when an accident happens in the tensile test.