The defect structures of steel mud pump air chamber casing was analyzed by using stereo microscope and metallographic microscope and also verified by three-dimensional solidification simulation software. The casting process of the air chamber casing was optimized and the two new improved casting processes were made by using the three-dimensional solidification simulation software. The new casting process has achieved sequential cooling, eliminated defects and improved surface quality. Steel air chamber casing castings were successfully produced which verified the effectiveness of the new process.