Abstract: Based on fick's first and second law and introduced surface hydrogen transmission coefficient "h",the mathematical modle of hydrogen escaping from deposited metal at a unsteady diffusion condition is founded in alcohol hydrogen test.The escape hydrogen-time function is caculated by the modle with the parameters of the initial hydrogen concentration,diffusion coefficient,surface hydrogen transmission coefficient and radius of the specimen.The results show that the surface condition of the deposited metal has great effect on the hydrongen escaping from the metal,which is a vital facter to be taken into account in the research of the hydrogen diffusion and accumulation in welded joint.The practicability of the modle is proved by succeeding in fiffing measured escape hydrongen-time curves and explaining some experimental phenomena.