Taking the deep hole machining of a mine diesel engine injector as an example, the influence of cutting speed, feeding speed and cutting fluid pressure on roundness of deep hole was studied by single factor experiment.Based on Box-Behnken experiment of three factors and three levels, the quadratic regression model of roundness was established. The optimal process parameters were obtained by variance and response surface analysis, and verified by experiment. The results show that the optimal processing parameters are cutting speed of 4 600 r/min, cutting fluid pressure of 11.5 MPa and feeding speed of 75 mm/min. The research results have guiding significance for the reasonable selection of processing parameters and the improvement of deep holes roundness.