Abstract: In order to study the change of gas leakage behavior caused by hydrogen mixing and rapidly realize the leakage detection of hydrogen-mixed gas, a plane leakage model for specific semi-closed places was established with the Fluent software, and thereby, the diffusion of hydrogen-mixed natural gas at different wind speeds and the dynamic development law at different volume fraction of mixed hydrogen were explored. The research results show that: the increase of wind speed will prevent the hydrogen-mixed natural gas from upward diffusion and accelerate the gas accumulation in the horizontal direction, while the increasing volume fraction of mixed hydrogen will reduce the gas diffusion height and slow down the growth of diffusion height. According to the relevant standards and the simulation results of height at which natural gas with different volume fractions of mixed hydrogen diffuses to its lower limit of explosion, the layout plan of flammable gas detectors of hydrogen-mixed natural gas pipeline in the semi-closed space scenario was proposed to ensure that emergent response could be provided immediately after leakage, thus assuring the safe operation of pipelines.