A multiphase relaxation calculation method with the adaptive process was established to analyze jetbuffer mechanism in water hammer with liquid column separation． The rationality and stability of this calculation method were verified by air-water two-phase shock tube． The internal flow mechanism of water hammer with liquid column separation was analyzed． The results show that for water hammer with liquid column separation whose secondary pressure variation value is higher than the theoretical ones，it can be divided into the direct water hammer， the buffer water hammer，and the jet water hammer． In the stage of buffer water hammer，the presence of bubbles reduces the pressure impact on the boundary，causing the water hammer wave appear buffered and the change of pressure value is smaller than the theoretical ones． In the stage of jet water hammer，the jet acceleration generated during void compression and collapse does not directly affect the pressure impact on the boundary，but it causes a secondary pressure change accumulation in the initial value after back propagation，and the change value is higher than the theoretical calculation value． The occurrence of jet water hammer affects the accuracy of value calculated by direct water hammer method，so this is the most noteworthy part of water hammer with column separation calculation．