Abstract: S195 diesel engine are widely used in many fields in china. Generally its quality is better, but there are also some drawbacks in it, such as the operation life of main bush and connecting rod bush, the oil leakage of bearing cap and the working life of cylinder cushion still should be improved. In order to solve these problems, the modal analysis of the cylinder block of S195 diesel engine was made and its deformation excited by force was calculated. The results showed that the difference of deformation at the interface of cylinder head is rather large and so is the deformation at the end face of main bearing hole in the side of flywheel. In order to reduce these deformations, the method of rigidity improvement was employed. Based on the original modal parameters, calculated and analyzed by the software of structural dynamic modification, and considered to the block structure and our experience several optimal methods were obtained. The results of before modification and after modification were listed in Tab.1 and 2, showing that after modification the deformation at the interface of cylinder head and the deformation at the end face of main bearing hole were reduced remarkablely. The new cylinder block was reanalyzed. The rigidity was improved apparently. The real use of the modified diesel engine showed that the drawbacks mentioned above were eliminated.