Based on the Boit-Savart law，the concepts of effective radius，effective spatial location and effective spatial orientation are introduced by comparing the far-field approximate calculation of charged current loops and the dipole magnetic field with the strict calculation of the magnetic field of the solenoids. By optimizing the solenoid structure and rebuilding，the computing model can overcome the limited length effect of the spatial magnetic field brought by the solenoid itself effectively. By simulating the system within the effective region，the result shows that the far-field approximate calculation of the dipole magnetic field not only meets the accuracy in magnetic field calculation but also has very fast calculation speed. The method can realize the magnetic calculation with the complex structure from many solenoids in real time and satisfy the real-time requirement of spatial positioning and electromagnetic navigation.