Abstract: The well cementing accident in one well was comprehensively investigated and studied. It was found that the import casing string was pulled out from the 5th casing mill connection end during well cementing process by analyzing the CCL curve of magnetism orientation survey well and distinguishing casing remains fished out from the well down. The possibility that over load resulting in casing connection pulling out could be eliminated because the load borne on the casing connection was far less than its rated tension strength according to the test results and casing service condition. It was considered finally that the casing connection was unscrewed first due to jarring load borne on it during well cementing because it was not made up with enough torque in the mill, and then was pulled out by analyzing and calculating.