The three-dimensional equal-diameter coil is a three-dimensional Helmholtz coil developed exclusively by our company. It has the same equivalent diameter of the coil on each axis, the number of turns, and the wires of the same specification. This type of winding makes the three-dimensional equal-diameter coil have the same resistance, inductance, uniformity and B/I coefficient for each axis; the coils of each axis are fixed to each other in the form of inlays. This mechanical structure gives it a small appearance and internal volume. The characteristics of large space; the overall frame is made of non-magnetic PMMA material, which is suitable for related research of low and weak magnetic fields, and is very suitable for use in shielded rooms and shielded cylinders. The highest magnetic field of the three-dimensional equal-diameter coil can be designed to be 0~5Gs.