Abstract: The response characteristics of P-wave velocity in abnormal over-pressure shale sand and carbonate rock can be interpreted by using experimental methods now,but the research on mechanism is still in the exploratory stage.Based on Biot theory, P-wave velocity equation was established and the major components of the P-wave velocity were analyzed.The main factors affecting P-wave velocity change were discussed and the variation features of P-wave velocity in abnormal over-pressure formation were explained.The results showed that in the abnormal over-pressure formation,the rock framework bulk modulus of shale sand rock became significantly smaller,that of matrix carbonate rock kept essentially constant,and that of fractured carbonate rock dropped slightly.The study showed that the response characteristics of P-wave velocity of shale sand rock and carbonate rock with abnormal over-pressure were determined by the rock framework bulk modulus,and the P-wave velocity of rock framework could be used to estimate the P-wave velocity variation trend of saturated rock.