Parallel-mechanism-based ultrasound robots bring the benefits of convenient robot-assisted remote ultrasound or automated scanning. During the use of this type of robot, accurate tracking and feedback of 6-DOF posture are critical for potential intelligent control and real-time navigation. However, this presents significant challenges due to the complex forward kinematics and environmental changes, making it difficult to apply in practical settings. To address this issue, we propose a novel 6-DOF pose tracking method based on an inertial measurement unit. This method directly senses the orientation and calculates the position using the intrinsic structural and kinematic characteristics of the parallel mechanisms. Therefore, it is possible to solve all posture information by only partial posture information. We conducted both simulation and physical experiments to evaluate effectiveness and robustness of the proposed method. The maximal tracking errors for the physical experiments are within the 2 mm and 0.6.°. The experimental results show promising performance, demonstrating the method’s potential for practical applications.