Abstract: Nowadays, the development of efficient and clean low-carbon energy equipment is the main direction of energy and power development in China. Currently, diesel fuel is generally used as the main fuel for generator units, which has higher particulate matter and hydrocarbon content in internal combustion engine diesel emissions compared with natural gas. After the conversion of the automotive diesel engine into a gas-fired internal combustion engine, the exhaust temperature is higher due to its long operating time. In order to further improve the fuel economy and increase the reliability of an exhaust manifold, this article optimizes the exhaust manifold structure by means of a 3D simulation modeling. Before the modification, the exhaust manifold of the original machine is designed with a short diameter in order to take into account of the performance at a low speed. In addition, the exhaust back pressure at a speed of 1 500 r/min is high, which is not suitable for the modified gas powered internal combustion engine. Through GT-Power one-dimensional numerical calculation, gas consumption in a single working condition can be reduced by optimizing the pipe diameter. After the manifold diameter increases from 35 mm to 40 mm, the power of the whole machine increases by 2.3 kW, the pump gas loss reduces by 5 kPa, and the gas consumption rate reduces by 1.7 g/(kW·h). Secondly, by using the inverse modeling method and Fluent UDF module, a transient fluid-structure coupling simulation model is established, and the heat transfer coefficient of the fluid domain wall and the heat deformation of the exhaust manifold area at three time points in five working cycles are calculated. The coupling time points are 180° CA, 540° CA and 720° CA. Based on the numerical results, two structural optimization methods for exhaust manifolds are proposed. The first optimization plan removes part of the stiffeners and connecting plates, and the second plan adds two stiffeners. After the heat load analysis and comparison in the transient process of temperature discharge, the results show that although the first scheme can save material use, the thermal stress is not obviously improved. The second scheme of adding reinforced bars can further reduce the amount of thermal deformation. The maximum thermal stress at the three coupling calculation points reduces by 2.7% compared with the results before the modification, and the amount of thermal deformation at the bending and joint of the tube reduces by 10%. The stiffener structure can restrain the deformation of the manifold to some extent. Overall, the optimization schemes are beneficial to improving the service life of the exhaust manifold and meeting the design requirements of gas-fired internal combustion engines. The modeling and optimization methods in this article can provide reference and guidance for the design of reformulated exhaust manifolds for gas-fired internal combustion engines.