The eccentric mechanism of the mechanical automatic vertical drilling tool can not always be held stable on the low side of the wellbore in the actual drilling conditions due to its own inertia and the influence of the external environment， resulting in the reduction of deviation correction accuracy. In order to study the change law of the angular position of the eccentric mechanism in the drilling process， a non-contact angular position measurement method based on the absolute magnetic encoder is proposed in this paper with the measurement data acquisition interface constructed， and the laboratory verification and reliability analysis carried out. The measurement error is less than 3 degrees， indicating that the non-contact angular position measurement method can monitor the angular displacement and the error is within the allowable error range. It can provide reliable data for the subsequent research and development experiment， and optimization of the mechanical vertical drilling system.