In a wireless magnetic induction communication system, the magnetic field distribution of the current-carrying coil affects the communication effect between the communication transceiver and receiver. In the study of magnetic field distribution, it was found that magnetic induction intensity and magnetic flux were important parameters to measure the effectiveness of communication. Aiming at the circular coils with rectangular cross-section of any turn numbers, this paper proposed an improved algorithm to calculate the magnetic induction intensity at any spatial position based on Biot-Savart law. At the same time, the calculation formula of the magnetic flux at the receiving point was also given. The coils were modeled and simulated with COMSOL software. The correctness of the improved algorithm was verified and compared with the traditional formula and simulation results, especially in the near field, which provided an important theoretical support for the further study of mutual inductance in the wireless magnetic induction communication system.