Assembly stress distribution and state are the main factor of the quality of largebolted joints in machine tool.Therefore,an assembly stress test method based on strain test is proposed.There was no need to damage the surface of the workpiece by this method.The relationship between the threedimensional (3D)rosette gauge test data and the surface principal strain was built up based on theanalysis of free surface stress state.Then according to Hooke’s law,the calculationformula of principal stress was established.By taking the spindle box and the guide rail slider bolt assembly of a machine tool plant as research objects,assembly stress distribution and state were researchedof different assembly process for bolt assembly.Theresults of this experiment validate the effectiveness of the presented method.