The deep water zone has great oil and gas potential,and hence it is treated as key field of exploration.The great variation of water depth, the roughness of seafloor and the complex geological setting make it difficult to conduct seismic imaging of the target object.Velocity building is one of the greatest difficulties.The rich refraction wave in the OBS (ocean bottom seismometer) data was used to invert the velocity model with the method of travel time inversion.The OBS was acquired by Guangzhou Marine Geological Survey in the east part of Pearl River basin on the north slope of South China Sea.The OBS data have good quality with refraction wave clearly observable.The imaging result shows that in the region of rugged seafloor, first arrival travel time inversion can provide the background velocity model needed by seismic imaging.Limited by the high frequency approximation,the velocity model has low resolution.More advanced full waveform inversion can provide high resolution model,with first arrival travel time inversion result as the initial model.