According to BEPCII project in IHEP, Superconducting solenoid magnet, installed on BESIII detector, is needed to provide a strong magnetic field and measure basic particles momentum. Magnet system is one of the important parts in BESIII detector. It can provide an axial magnetic field of about 1.0T with a good field uniformity over the tracking volume. Main Drift Chamber will measure the trajectories of charged tracks emerging from the collisions in the field, then do some research about interaction and law among basic particles. The iron absorber plates of the muon detector not only provide the magnetic flux return, but also support the global BESIII detector.

Based on the physics requirements, the main drift chamber should have a high momentum resolution which is decided by the multiple Coulomb scattering. In this situation, it is impossible to improve momentum resolution by improving space resolution and increasing measurement times (increasing layers of sensitive wires), but increasing the magnetic field can improve momentum resolution. On the other hand, if the magnetic field is too high, more low-momentum particle would circle around inside the drift chamber and become difficult to measure. Considering all factors, we selected 1.0T as the field of BESIII solenoid magnet. More details can be gotten from design report.