Abstract: The determination of strength parameters of rock and soil has always been the difficult problem in the geotechnical field, especially for soft rock. The relationship between stress and strain of soft rock is illustrated and reliable strength parameters is presented, by means of in-situ shear test in the soft rock foundation of Jiyang nuclear power plant and combined with laboratory three-axial test. The results show that: (1)Apparent stress yield and plastic deformation exist in the process of horizontal shear of Tertiary soft rock samples; (2)Each in-situ soft rock sample shows complete curves of stress-strain relationship, including linearly elastic part, elastic-plastic part, peak part and strain softening part; (3)Tertiary rock in Jiyang nuclear power plant, whose deformation characteristic is more similar to soil, is rock with plastic failure characteristic and the shear strength parameters can be determined by the yield strength of in-situ shear test, i.e.c=19.535kPa and =33.07; (4) obtained by in-situ shear test is equivalent to that obtained by three-axial test, while c obtained by in-situ shear test is a bit less than that obtained by three-axial test, and the former c and are more representative, considering that in-situ shear test reflects the actual situation better.