Based on the instability mechanism of wave formation at the interface in explosive welding and fluid-elastic-plastic model, the conditions for Rayleigh-Taylor and Kelvin-Helmholtz instability were found between fly plate back and air. Therefore, the instability between metal fluid foil and air was proposed. It can be deduced from the ratio of amplitude and wavelength that linear relationship is proper. Then we established double layer fluid flow instability linear control equations, and gained expression of instability development exponent. Results show that at the competition between metal fluid elastic-viscosity and Helmholtz-Taylor synthesized instability, disturbances of special wavelength are prior developed, while others are attenuation or have no time to develop. Consequently, the wavelength and amplitude of welding interface along detonation direction is uniform. In addition, experiments also validate this conclusion