Abstract: When a rock sample is extracted from the underground rock mass, the interior stress of the rock sample will release and its mechanical properties will change. The unloading effect will happen. According to the experiments on P-wave velocity and stress designed by the authors, it is found that the P-wave velocity vs. stress curves (V-S curves) of rock samples can reflect the three stages of the bulk distortion during the loading process. The total V-S curve can be divided into two parts. When the stress is less than sc, that is, one third to one fourth compression capacities of rock samples, the P-wave velocities of rock samples increase rapidly with the stress and the V-S curves can be fitted with power function. When the stress is larger than sc, the P-wave velocities of rock samples gradually reach the peak value, then they dramatically decrease. The V-S curves can be fitted with conic function. During the V-S experiments, the increasing velocity of the P-wave velocities will dramatically decrease if the stress reaches the stress which is undergone by rock samples in the field. The unloading effect index which is the ratio of the P-wave velocity with the initial geo-stress to that without stress is put forward according to the V-S relation. After the rock samples of gneiss and diorite from Huangdao water sealed underground oil tank are tested, the calculated unloading effect indexes of the gneiss and the diorite are 1.13 and 1.08 respectively.