Abstract: Drop tower is an important facility to obtain microgravity environment. For the scientific experiments which will be performed in drop tower, it is quite important to know the residual acceleration of the drop capsule. Laser interferometry is a new method for measuring the residual acceleration of the drop capsule. The basic principle of this method can be described as following: a reference object falling in the drop capsule can fall more freely than the drop capsule itself because the outer air drag acts on the latter, so there is a difference of acceleration between the reference object and the drop capsule. The difference of the acceleration can be obtained according to the relative displacement which can be measured by laser interferometry. The main disturbance of this method is discussed in details. The total error caused by the main disturbance is 1.2 x 10^-7 g, less than the estimated microgravity of 10^-4~10 ^-6 g, so the laser interferometry is a suitable method for measuring the residual acceleration of the drop capsule.