The anchor-girder composite system is one kind of reaction methods used frequently in rock mass in-situ test, but the force on rock mass caused by the anchor will lead to a certain degree of bias on the deformation accuracy of rock mass. Through establishing the theoretical model of force on rock mass caused by anchor, the theoretical formula of deformation on test points caused by anchor was deduced. Compared with the theoretical deformation of rock mass, the formula on the degree of influence caused by contact pressure in rock mass in-situ test was given. Combining with the in-situ test, according to mechanics parameters of a certainⅡkind of rock mass,the theoretical deformation and reaction-caused deformation were calculated separately. The results show that for thisⅡkind of rock mass, the reaction-caused deformation is 0.035% of the theoretical deformation, so the theoretical foundation was provided for the enhancement of accuracy of the test results.