Magnetic flux leakage detection (MFL) technology has been widely used in the field of ferromagnetic material defect detection, in which the magnetic dipole method is the most widely used mathematical method to predict magnetic flux leakage from structural defects. The magnetic dipole method is mostly used to predict the magnetic flux leakage signal of regular defects. For complex defects, the existing unit magnetic dipole band superposition model can predict the magnetic flux leakage signal, but because the magnetic charge density of complex defects required by the model calculation is unknown, and its distribution is uneven, the calculation is complex. Therefore, this paper proposes a calculation method of discrete magnetic charge density field, which can quickly calculate the magnetic charge density of three-dimensional irregular defects, and directly predict the magnetic flux leakage signal combined with the unit magnetic dipole band superposition model, so as to reduce the computational complexity and quickly and accurately obtain the magnetic flux leakage signal of three-dimensional irregular defects. The method is applied to the prediction signal obtained by the unit magnetic dipole band superposition model and the COMSOL software simulation signal, which shows that the method is feasible. The experimental results show that the prediction performance of the unit magnetic dipole band superposition model is significantly improved compared with the original model, the maximum prediction error is reduced by 90.08%, and the calculation time is reduced by 97.43%, which provides a fast and effective solution for the calculation of the magnetic charge distribution of three-dimensional irregular defects.