Taking a low-speed two-stroke marine diesel engine bedplate as the research object.The main bearing seat（material ZG200-400） crack growth rate was tested,and the fatigue crack growth rate and the Paris formula of the stable expansion section were obtained.The three-dimensional crack propagation life analysis model of the main bearing seat was established by the sub-model method.The real boundary conditions of the main bearing seat was obtained by combining with the analysis results of the alternating stress of the bedplate under the actual operating conditions of the engine（360 degree crank angles）.The crack growth life of the main bearing seat with initial crack was calculated.The analysis results show that the crack growth life is less than that of the low-speed two-stroke marine diesel engine overhaul inspection interval.The research results of the thesis provide an reference for the damage tolerance in design engineering application.