An Inline 4-cylinder engine is equipped with second-order balance shafts.When the engine is running under full load in 5000rpm,the engine generated severe structural radiation noise.The bench test analysis shows that the main reason is the resonance of the engine near 800Hz and 1500Hz. In this paper, a method for modeling and analyzing the vibration of the engine structure is proposed, and the sound quality of the engine is evaluated and imporved by the Moore–Glasberg loudness method. Firstly, the finite element model of the engine was established, and the experimental modes of the engine casing assembly, crankshaft and balance shaft were measured. The natural frequencies and modal shapes obtained by calculation and experiment were compared, which validates the established finite element model.Secondly, a flexible multi-body dynamic model of the engine was established. The inertia and torsional stiffness of the drive shaft at flywheel nose are taken into consideration in the model, and a modeling method of the scissor gears on the balance shaft is proposed. The calculated and measured torsion angle at crankshaft nose were compared and analyzed. The results show that the average relative error between the calculated value and the measured value of the torsion angle at crankshaft nose is less than 10%, which verifies the flexible multi-body dynamic model. The influence of the shim gear and drive shaft on the engine structure vibration was analyzed. The results show that the shim gear restrains the knock excitation of the engine, and the drive shaft reduces the torsional vibration of the crankshaft. Thirdly, taking the vibration velocity from engine casing assembly surface as the boundary condition, the field point sound pressure level of the engine casing assembly were calculated.The calculation results show that the overall noise of the engine is serious around 800Hz and 1500Hz, and the calculation results and the test results are in good agreement. In order to improve the sound quality of the engine, the Moore–Glasberg method was used to calculate the loudness of the engine field points, and an engine structure re-design method to enhance sound quality is proposed.