Abstract: When a multi-legged wall-climbing robot attaches to a wall with negative pressure adsorption, the underside of each suction module is required to be parallel to the wall surface. Therefore, the posture of a suction module with respect to the wall surface should be detected and controlled. To solve this problem, a non-contact measurement method based on distance sensors is proposed to detect the relative posture of suction modules. After a brief introduction to the biped wall-climbing robot W-Climbot, the theoretical model for computing the relative posture (the distance and the inclined angle between the suction module and the target surface) of a suction module is presented, and the conditions of reliable adsorption are analyzed. Then the corresponding suction modules with the function of autonomous posture detection are developed and tested with experiments. According to the measurement results, the detection error is analyzed and the experimental data are amended. Experiments show that the method is simple and effective, with the ranging precision of 1mm and the angle measuring resolution of 0.3°, which meets the practical utilization requirement of autonomous climbing with a biped wall-climbing robot, and may be an automation solution to relevant industrial applications involving vacuum adsorption.