We apply cross-hole seismic tomography cascade algorithms to obtain accurate and high resolution P wave velocity structure in Ken 71 block. At first, for precisely picking the first-arriving travel times, we compose the break first wave filed according to take off angle of receiver recording. And then, a multi-layered velocity is structured consisted with well sonic logs as the starting model. Finally, we perform cascade algorithms. This method is characterized by the idea of successive approximation from low to high resolutions. The BPT algorithm of low resolution and stability was used first to provide initial earth model for higher resolution algorithm: improved algorithm of LSQR with damping iterations. The high resolution tomography results are matched well with borehole logs, the distribution of the high/low velocity layers in tomography image are consistent with the location of sand/mud layers which are predicted from the borehole logs. The result can image sand-mud interbed with layers 1/4 wavelength. The research shows that cascade algorithms simultaneously considers the high resolution and accuracy of tomography and can significantly improve the quality of crosswell traveltime tomography. So this method could be widely used in exploration and monitoring of oil fields.