In order to ensure the secure and stable operation of the main branch temporary cofferdam built by the tail project of the lower reaches of the Ganjiang River，the real-time monitoring system of high density resistivity method is used to detect the cofferdam dam body non-destructively，and the continuity of the cut-off wall and the possible safety hazards were identified. The test results show that the internal stratification of the whole cofferdam is obvious，the vertical bedrock interface and the horizontal interface is clear，and the continuity of the dam foundation cut-off wall is preferable，which meets the design and construction requirements. It also shows that the resistivity method can better reflect the electrical structure inside the dam body.