To save the semi-active suspension electrical architecture and ECU development process, a new semi-active suspension sensor layout scheme is proposed, and the optimal IMU installation position is given. An accurate and complete formula for calculating the acceleration of four sprung positions by IMU signal is established. The proper angular rotation order and reference frame are selected to ensure the correctness of the method. The influence of sensor mounting position on soft sensing accuracy is given by using optimization theory and partial derivative method combined with different suspension weights, and the optimal mounting position with constraints is given. The experimental results show that the new sensor arrangement scheme and kinematics solution method can well predict the vertical acceleration of four sprung positions and have high stability, but the solution accuracy of different positions is quite different. The experimental results of the constrained optimal installation position obtained by the optimization theory are consistent with the theory (the root mean square error is about 0.6 m·s^-2). Compared with other installation positions, the experimental results of the optimal installation position have higher stability and measurement accuracy for different working conditions. Compared with the horizontal installation position, the vertical position of IMU has a greater impact on the final solution results and accuracy. An equivalent error ellipsoid of IMU installation position is given with combination of quantities and spatial forms, which can analyze the installation position more clearly and intuitively.