Abstract：The magnitude of the maximum principal stress is determined by using acoustic emission measurements of cores at 301, 598, 1003, 1258 and 1531 m depth in the main borehole of the Chinese Continental Scientific Drilling (CCSD) Project, and the measurement results of acoustic emission are compared with the measurement results of borehole breakouts. It is shown that the results from two methods are in good agreement. The results of stress measurement are reliable. The magnitude of stress in the main borehole increases with depth. The maximum principal stress is 13.4 MPa at 301 m depth and 55.2 MPa at 1655 m depth. The increase rate with depth is 0.0279 MPa/m. The direction of the maximum principal stress is N 54.2°±3.3°E and does not change with depth. It is considered that acoustic emission combined with borehole breakouts is an economic and effective method for deep in-situ stress measurement.