Compared with other multi-cylinder engine, 3-cylinder engine has a more complex balancing problem. The balancing shaft design was generally used to offset the unbalanced torque during the operation of engine, but the engine structureal strength design would be challenged after the balancing shaft was used. Taking the structural strength of a 3-cylinder engine upgraded model as the research object, the dynamic model was built to acquire the working load of engine block, the finite element model was constructed to conduct the stress analysis of balancing shaft and cylinder block, and the fatigue strength was further verified with the fatigue software. Finally, the weak position determined by the simulation was verified by the strain test and the reliability of engine structureal strength design was guaranteed.