The piston is a key moving-part of the reciprocating piston compressor with a high failure rate. Because of the difficult exact calculation of machinery dynamic, the traditional and common way to find the reason of fracture is based on metal-lographic phase of piston material. The C102B reciprocating piston compressor is an important equipment in methanol plant. In this paper, in connection with the piston fracture of the C102B fresh air compressor on service , besides a lot of analysis on metal-lographic phase, the 3-D model of piston and cylinder were built-up, and the two dangerous places where the piston suffered max-force were found. Bases on ANSYS and FEM technology, the value of max-force on piston at the two places were carried out. Finally, the reasons of piston fracture of the C102B fresh air compressor were found and some preventive suggestion was proposed.