The weak strain induced by uniaxial strain device is calibrated by strain-induced second-harmonic generation (SISHG) from silicon (111) surface. Dependences of the strain-induced second-harmonic intensity on sample azimuth angle show that the strain leads to increase of SH intensity. The high consistency of the SH-measured strain and the applied strain indicates that weak strain can be accurately calibrated by SISHG. The small applied strain does not greatly affect the 3 m symmetry of silicon (111) surface, but enhances the SH intensity evidently. The bulk inversion symmetry of crystal silicon vanished under applying of uniaxial strain and this also has demonstrated by first-principles simulation. Furthermore, the theoretical relative variation of Si–Si bond length agrees exactly with the applied strain along [111] direction.