In underground engineering, especially in deep excavation engineering, in-situ stress is one of the important parameters of the guiding excavation, thus the measurement of in-situ stress is crucial. The CSIRO in-situ stress measurement method is recommended by the International Society for Rock Mechanics. The deep granite of the Sanshandao Gold Deposit was processed into standard cylindrical specimens. The Hoek-Brown strength criterion curve was obtained by uniaxial and triaxial compression tests. A single rock sample multi-confining pressure step loading test considering multiple confining pressures was designed. Physical parameters such as elastic modulus and Poisson’s ratio were obtained by calculation of experiment data. Deformation parameters including bulk modulus K and shear modulus G were introduced, and the mean stress increased following a non-linear manner. The constitutive relation between the bulk modulus and shear modulus vs. mean stress was fitted by the hyperbolic function K/G=K₀/G₀+Pb+a×P, where K₀/G₀ is the initial modulus of deformation, K₀/G₀+1/a is approach value of mean stress at limit, 1/b is initial growth rate of deformation modulus, all of which have clear physical meanings. Based on this formula, the high-pressure biaxial formula in the CSIRO in-situ stress measurement method which considers the non-linear elastic deformation of rock was deduced, and its applicability was verified by rock core removal high pressure biaxial loading test conducted on the deep granite of Sanshandao Gold Deposit.