Based on Halbach permanent magnet array, a method for optimizing the magnetic circuit of the adsorption structure under a limited volume was proposed. Aiming at the problem of single function of ship wall maintenance robot, a modular and multifunctional ship wall maintenance robot was introduced, including a cleaning module, a rust removal module and a painting module, and the adsorption structure of the moving mechanism was optimized and analyzed. The influence of planar Halbach permanent magnet array on adsorption force was studied. The primary and secondary effects of different parameters of planar Halbach permanent magnet array such as N,n,ε1,ε2,h,h1 on adsorption force were analyzed by orthogonal experiment, and the optimal scheme was selected. The experimental results show that the yoke structure should not be added when the height of the permanent magnet array is fixed; when the length L of the permanent magnet array is less than or equal to 55 mm, parameter N should be set to {2,3}; when width W is less than or equal to 20 mm, n should be {1,2}; the values of ε1 and ε2 are affected by N, n and h.