Abstract: The algorithms and numerical schemes of some usual external magnetic field loading modules were introduced and studied in detail for the 3D fully electromagnetic and PIC simulation code NEPTUNE. The loading of simple magnetic field distribution function and discrete magnetic field data, the loadings of magnetic field distributions produced by current elements of direct and helix lines and solenoid, and the loadings of magnetic fields produced by wiggler and permanent magnet were all included and discussed thoroughly. All the calculated results validate the credibility of these external magnetic field loading modules. As a practical example, a coaxial diode with electron beam emission under different guide magnetic fields was test. Its emission characteristics validate these field loading m