In order to solve the problem that magnetic sensors are difficult to collimate when measuring the core current of the three-core power cable, a method for determining the declination angle of magnetic sensors is proposed. Four magnetic sensors are used to measure the tangential components of magnetic flux density at four different locations on the surface of a three-core power cable. The "scanning" algorithm is adopted to eliminate the nonlinear influence of the declination angle of magnetic sensors in calculation process, and the nonlinear equations are converted into linear equations for solving. Based on the measured value of magnetic sensors and the design parameters of the three-core power cable, the declination angle of each magnetic sensor installation position and the corresponding core in the cable are obtained, and the current of each core of the three-core power cable can be solved. The test results of electromagnetic field finite element simulation and analytical model calculation show that this method has good accuracy, and it has guiding value for the effective and accurate monitoring of the operation state of the three-core power cable.