Abstract: According to the heat balance equation,the solidification heat-transfer law and the physical properties of metals of the inner and outer layers of the pipe,the model of the relationship between melting point difference of inner and outer metals and the thickness ratio of fusion layer of the bimetallic composite pipe is established. It is reveals via the relationship formula and the relationship curve that the melting point difference ΔT to the fusion layer thickness ratio Δd_o/d_o is proportional,and that along with the Δd_o/d_o(from 0.02% to 0.20%) gradually increases,the maximum allowed value of the ΔT is also increases gradually.When the maximum thickness ratio of the fusion layer gets up to 0.20%,the allowable range of ΔT of component metals can be obtained from the above mentioned relationship formula.