Abstract:The method of approximate velocity inversion in the paper "time difference of time-lapse in layered medium" (simplified as "time difference" in the follows) published by《Oil Geophysical Prospecting》 is worth discussing.The paper gave accurate formula of direct velocity inversion.The author considered by theoretical study that the raypath corresponding to same shot point and receiver point is unique in uniform horizontal layered medium.The model test showed that the calculated result of time difference based on shot point and receiver point is generally larger,but the rule of variation reflected in time difference is coincident to qualitatively analyzed result in the paper.