Abstract: The study of directional rotary steerable drilling tool started relatively late in our country and many key parts are still in research stage. The measurement of relative angular position of bit axis (eccentric shaft) and drill collar of the system is the key and prerequisite for drilling tool maintaining a relatively large stable tool face during steering, so its accuracy and instantaneity are very critical. Based on the research on tool structure and tool face measurement principle of directional rotary steerable drilling system, contrastive analysis was conducted to the applicability of arctangent method and phase discrimination method, which are two kinds of angular position measurement methods using resolver. Two corresponding algorithms are simulated and confirmatory tests are carried out for phase discrimination method in lab. The simulation and test results prove that, in the case of simulated downhole motor rotation and interference, phase discrimination method has higher stability in signals treated by arctangent method, and can track in real-time and reduce in high precision the rotation angle of tool eccentric shaft relative to drill collar, providing basis for tool face angle measurement, steering control and development of the entire tool.