In this paper, ADAMS multi-body dynamics simulation was introduced in order to solve the diesel engine piston pin dynamics. Take diesel engine piston for example, the model of piston system was established based on ADAMS/Engine module. According to rotational speed of crankshaft, piston pin dynamics simulation was analyzed. Through Fast Fourier Transformation, piston pin power spectral density was got for piston pin fault diagnosis. The results show that simulation results are consistent with theory, dynamics simulation applied in fault diagnosis is feasible.