Abstract: Amplitude-preserving or waveform-preserving imaging results are the basis for reservoir characterizing and parameter estimating in lithologic reservoir exploration. Kirchhoff integral pre-stack time migration and F-K domain wave equation pre-stack time migration is not suitable for the medium with gradual transverse change in velocity. Therefore, an offset plane wave finite-difference pre-stack time migration method was proposed. The CMP gather plane wave decomposition was discussed. Offset plane wave equation was derived and its finite-difference solution was given. Then, an approach for generating angle imaging gather is deduced. A velocity model with gradual transverse change from Daqing oilfield was used to verify the fidelity of the method. The results show that the method is suitable for the medium with gradual transverse change in velocity, can preserve amplitude of field extrapolation operator and provide better basic data for sequent AVP/AVA analysis.