The diagnosis and treatment effect of the swallowing wireless magnetic capsule robot in gastrointestinal tract is closely related to the external magnetic driving force of the robot. In this article, the theoretical model of external magnetic driving force is formulated. The mathematical expression of the interaction between two diametrical magnetizing ring permanent magnets is deduced which is based on the equivalent magnetic charge method. The numerical calculation is carried out by the adaptive recursive calculation method. In addition, a real-time measurement method of magnetic force between ring permanent magnets is developed, a magnetic-spacing synchronous measurement instrument is developed to carry out experimental research, and a finite element simulation model of 3D ring permanent magnets is formulated. The experimental measurement of the magnetic-spacing relationship is in good agreement with the theoretical calculation and finite element simulation results. The error is less than 4% , which verifies the accuracy and reliability of the theoretical model and the finite element simulation model. The influence of length, thickness and volume of embedded permanent magnet in the capsule robot on magnetic force is revealed by parameter analysis. The results provide a theoretical foundation for precise driving of capsule robot in gastrointestinal tract.