For fracture cracks that occurred in the tight coupling exhaust manifold durability test of a four-cylinder gasoline engine with EGR channel, causes and solutions for fracture failure were found with the help of CFD and FEA numerical simulations. Wall temperature and heat transfer coefficient of the exhaust manifold inside wall were first accurately obtained through the thermal-fluid coupling analysis, then thermal modal and thermoplastic analysis were acquired by using the finite element method, on account of the bolt pretightening force and the contact relationship between flange face and cylinder head. Results showed that the first-order natural frequency did not meet the design requirements, which was the main reason of fatigue fracture. However, when the first-order natural frequency was rising, the delta equivalent plastic strain was increasing quickly as well. Ultimately, to solve the problem, the semi-shell was strengthened and some dents of critical areas were added so as to absorb some energy, consequently, the plastic strain decreased in the process of thermal expansion and cooling contraction. Meanwhile, the optimum model was verified effective by test. It can be concluded that this study was of certain value for engineering application.