The invention relates to a gas valve internal leakage on-line diagnostic method. The problems of difficult on-line diagnosis and difficult internal leakage level quantification index providing in the prior art are solved. According to the gas valve internal leakage on-line diagnostic method provided by the invention, according to acoustic emission characteristic parameters representing gas valve internal leakage, the function relationship between the characteristic parameters and gas valve internal leakage rate and upper and lower limits of different internal leakage degrees corresponding to various kinds of characteristic parameters, a gas valve internal leakage diagnosis database is established; the database and an internal leakage acoustic emission data collection device form a gas valve internal leakage on-line diagnostic system; and according to the internal leakage level distinction basis, the technical scheme of gas valve internal leakage on-line diagnosis solves the problems and can be used for gas valve internal leakage on-line diagnosis.