Nowadays, Magnetic and Inertial Measurement Units (MIMUs) have become useful tools for determining the attitude of moving objects. An Attitude Calculation Algorithm (ACA) is programmed into a MIMU, which determines the attitude by fusing three built-in sensors, namely, gyroscope, accelerometer, and magnetometer. Attitude is often represented by Euler angles, quaternion, or rotation matrix. Despite that great development has been achieved, how to eliminate the negative effect of magnetic disturbance on the pitch and roll estimation is still difficult for a ACA. This paper aims to propose an Extended Kalman Filter (EKF) based approach for problem solving. Inspired by the fact that the Euler angles are computed by using some elements of the rotation matrix, a Diagonalizable Regulation Matrix (DRM) is multiplied to the right half part of the gain matrix (which is multiplied to the residual of the magnetometer measurement in the update stage of EKF). The determination of DRM is based on the principle that these elements can be appropriately regulated by the DRM so as to make the magnetometer measurement only participate in the computation of azimuth. No further revision to EKF is made except the Kalman gain matrix, hence the proposed EKF behaves similar as traditional EKF, except its ability of eliminating the negative effect of magnetic disturbance on the pitch and roll estimation. The superiority of the proposed EKF is validated by comparing it with four other competing algorithms through simulation and experimental experiments.