Abstract: This paper develops a new apparatus for measuring the vertical gravity gradient based on the laser interferometry. The apparatus uses the method of dual optical interferometer to determine the time shifting coordinates of dual free-fall bodies spaced 50 cm vertically， and then calculates the gravity gradient by using differential algorithm. In design and development of this instrument， particular attention is paid to those aspects which would affect synchronistically automatic control of the free-fall bodies and vertical adjustment of the dual measuring beams. The results of the experiments show that the apparatus can obtain the interference signals of these two free-fall bodies during the freely falling and complete the measurement of the gravity gradient with accuracy better than 100 E.