The invention discloses a dilatometer for measuring the expansive force by a constant volume method, and relates to geotechnical test apparatuses. According to the structure of the dilatometer, a fixing system is connected with a measuring system, the fixing system comprises a fixing base, circular steel columns, a square crossbeam with a hole, a knob, a lower permeable stone, an upper permeable stone, a cutting ring fixing cylinder, a cutting ring, a nut, and a gasket, and the measuring system comprises a pressure sensor, a data display meter, a computer, and a dial gauge. The dilatometer is advantageous in that 1) constant volume: the volume of a soil sample in the whole test process is invariable; 2) high precision: the pressure sensor is advantaged by high precision and large measuring range, and the precision of test results is improved; 3) wide range: the dilatometer is applicable to measurement of the change of the expansive force in the soaking process of the soil sample and with the change of the environmental humidity condition and the test of the expansive force under the temperature control; and 4) the structure is good, the installation is convenient, the operation is simple, and the data display is visual.