With the aging of population, the number of Total Hip Replacement Surgeries (THR) increased year by year. In THR, inaccurate position of the implanted prosthesis may lead to the failure of the operation. In order to reduce the failure rate and acquire the real-time pose of Anterior Pelvic Plane (APP), a measurement system is proposed in this paper, which includes two parts: one is used to acquire the initial pose of the APP, the other is used to estimate the real-time pose of the APP. Both are composed of an Inertial Measurement Unit (IMU) and magnetometer sensors. The Extended Kalman Filter (EKF) is adopted to estimate the attitude of the measurement system, based on which, the real-time pose of the APP could be acquired together with the algorithm designed in the paper. The experiment results show that the Root Mean Square Error (RMSE) is less than 1.6 degree, which meets the requirement of the THR operations.