Ultra-high performance concrete （UHPC） combined with normal strength concrete （NSC） has excellent interface tensile strength， and the interface tensile strength is even more than substrate strength. Thus substrate failure is easy to occur in interface tensile tests， and the interface performance and its influencing factors can not be evaluated accurately. A method of substrate constraint strengthening is proposed to control the failure location at the repair interface， and accurate interface tensile strength is obtained. The effects of substrate strength and repair surface roughness on the interface tensile strength are analyzed， which overcomes the shortcoming of the traditional test method. The UHPC-NSC interface is denser than the NSC-NSC interface， the n（Ca）/n（Si） in the overlays transition zone is lower， and the width and porosity of the overlays transition zone are reduced by 91% and 70%， respectively.