The in-situ stress is a crucial parameter in rock engineering. CSIRO in-situ stress measurements are one of the four suggested direct measurement methods and have been widely used worldwide. With the development of exploration and excavation in deep earth, temperature disturbances during overcoring and the nonlinear behavior of rock under high stress levels increase the error of CSIRO in-situ stress measurements. To satisfy the requirements of in-situ stress measurements in depth, a new local digital wireless CSIRO cell was developed and is introduced in this paper. The twin temperature compensation method is illustrated, and deviations based on several data processing methods are compared as well. The thermal output of a rock-gauge set is discussed, and a series of self-compensating strain gauges are selected for the manufacturing of the CSIRO cell. Two sites for in-situ stress measurements were utilized at the Sanshandao gold mine (795 m deep) and Lilou iron mine (300 m deep). Strains resulting from stress relief are analyzed, and temperature deviations occurring in the sensor system and data logging system are calculated by the twin temperature compensation method. To eliminate the theoretical limitation of the linear behavior assumption, nonlinear relations of the mean stress vs. bulk and shear modulus are proposed for the biaxial pressurization test. A high confining pressure apparatus was developed to reach a similar stress level as deep rock; in addition, associated equations to describe nonlinear features of rock core are deduced. Biaxial testing at high pressure was conducted, and both linear and nonlinear fits were applied to overcoring cores of the Sanshandao and Lilou mines. The results show that the proposed nonlinear method achieves an extremely good approximation and higher precision.