Objective Mobile X-ray imaging device (C-arm) has become standard equipment in image-guide surgery due to its advantages of portability and real-time performance. However, most of these C-arm devices equipped in hospitals cannot implement rotation angle measurement. Methods In this paper, we design a rotational motion angle location method for C-arm device, based on the inertial measurement unit. Through fusing real-time inertial measurements from inertial measurement unit, rotational matrixes of the world coordinate system and carrier coordinate system can be derived. Then, the rotational angle of C-arm can be obtained utilizing of the quaternion component decompositions, which can be used to measure the rotational movement of C-arm. In order to verify the feasibility and the accuracy of the method, a calibration experiment of sophisticated electric rotating platform and a C-arm rotational positioning experiment are carried out. Results For the experiment of the electric rotating platform, the location error is less than 0.5 degrees. For the experiment of C-arm, the maximum positioning error is less than 0.8 degrees. These results meet the requirements of clinical surgical navigation (location error less than 1 degree). Conclusions This angle location method is simple, practicable and highly adaptable without requirements of calibration and correction positioning, providing necessary angle information for the subsequent surgical navigation.