This paper simulated and optimized the combustion chamber of the direct metering of the natural gas calorific value experimental platform. This material conducted a numerical simulation of methane combustion in the combustion chamber using ANSYS Fluent software, and the effects of different mixture inlet pipe length, combustion chamber diameter, and methane nozzle diameter on carbon monoxide emission concentration were studied. Aim to promote the complete combustion of methane, a more appropriate combustion chamber structure, and size were determined through carbon monoxide emission concentration. Through the comparison and analysis of the temperature field, velocity field, and concentration field of each component, it is suggested to set the length of the inlet pipe to 25 mm, and set the diameter of the combustion chamber and methane nozzle as 44 mm and 1.5 mm, respectively. After optimization, carbon monoxide emission concentration decreased from 27.9 PPM to 17.8 PPM, decreasing by 36.2%.