The invention discloses an underwater gas pipeline leakage diffusion experiment simulation device. The device comprises a gas conveying pipeline for conveying gas, a leakage water tank and a leakage pipeline horizontally penetrating the leakage water tank; the leakage pipeline comprises an outer pipeline and an inner sleeve which are coaxially arranged; the two ends of the outer pipeline are connected with the sealing pipe through a third movable joint and a fourth movable joint respectively; the two ends of the inner sleeve are respectively connected with the gas conveying pipeline through afirst loose joint and a second loose joint; an external leakage hole is formed in the external pipeline, and a plurality of internal leakage holes with different apertures are formed in the internal sleeve; and the hole diameter of the internal leakage hole is smaller than that of the external leakage hole. The device can be used for measuring different leakage holes and different leakage directions, the leakage characteristic of the pipeline when the pipeline leaks under different water depth working conditions is avoided, the experiment device is simple to operate, the instant switch of theleakage hole can be realized, and can really reflect the instantaneous leakage process of the gas delivery pipeline when the gas delivery pipeline leaks.