Abstract: The crankshaft of a reciprocating methane compressor ruptured during operation. The fracture cause of the crankshaft was studied by macroscopic and microscopic fracture morphology observation, chemical composition analysis, microstructure observation and mechanical property testing. The results show that the torsional fatigue fracture occurred in the crankshaft. Under the alternating torsonal stresses, the irregular and rough transition fillets of the crankshaft main journal and the rough machining marks near the oil hole produced stress concentration, leading to the initiation of microcracks. The severe banded tempered troostite, non-uniform grains and non-metallic inclusions in the structure caused the poor mechanical properties and accelerated the fatigue crack growth. It was recommended to strictly control the heat treatment and manufacturing process of the crankshaft and optimize the design of the crankshaft structure to prevent similar accidents.