In order to improve the stability of the adsorption force during the operation of the permanent magnet adsorptive out-pipe climbing robot, and to solve the contradiction between the adsorption capacity and the movement flexibility, the structure of the variable magnetic adsorption unit was studied. By establishing the finite element model of the variable magnetic adsorption unit, and analyzing the influence of the rotation Angle of the cylinder magnet and the armature gap distance on the magnetic adsorption force, the mathematical model of the magnetic force control function is fitted by MATLAB. Using this mathematical model to control the servo motor, the rotation Angle of the servo motor can be fed back at any air gap spacing, and then the magnetic adsorption force can be controlled to ensure the stability of the magnetic adsorption force in the process of robot crawling. The measurement experiment shows that the magnetic force control method is feasible and effective.