Bolt joints are the important part of the assembly in machine tool, and the uniformity of the assembly stress is the main assurance of the precise assembly. An assembly stress test method was used based on strain test, aiming at the single bolt joint, the law of influence was researched of the interface roughness on the bolt joint of stress characteristics. The experiment indicated that the inside stress characteristics of the workpiece could be expressed accurately by the test points set in the side face of the workpiece. The research result shows that with the increasing of the roughness of the joint interface, the compression strain in the direction along the bolt joint is increased when preloaded by the same stress.