In order to explore the influence of the inlet structure on the separation efficiency of a horizontal gas-liquid separator for natural gas and to optimize the design of the inlet structure to improve the separation efficiency and reduce the energy consumption. Through numerical simulation research, the four basic parameters of the inlet structure of the separator, namely the shape, height, inclination angle and distance from the inlet of the deflector, were compared, and the influence of different inlet structures on the flow field changes in the separator and their effect on the separation performance was analyzed. The results show that: the separation effect of concave deflector is better than other shapes. Higher height of the deflector can effectively slow down the inlet velocity, but when the height of the deflector is too high, the stability of the internal flow field of the separator is destroyed, which in turn reduces the separation efficiency. When the inclination angle of the deflector is 75°, it can effectively decelerate the inlet fluid, and create a stable cyclone to increase the retention time of the fluid in the separator chamber, thus improving the separation efficiency. It is recommended that the distance between the deflector and the inlet should be taken as large as possible without affecting the separation efficiency. Based on the simulation results of the aforementioned four parameters, a new type of horizontal air-liquid separator with an inlet structure was designed, and the separation efficiency was improved by 7% compared with the original structure. The research results can provide a scientific theoretical basis for the further improvement and practical engineering application of horizontal gas-liquid separators.