According to the unexpected falling-down of some mortar projectile loading sand the differences of shooting conditions and centroid position between the live ammunition and mortar projectile loading sand is analyzed. The derivative of torque coefficient is derived using static stability theory, and the flight stability is analyzed. On the basis of mortarprojectile point mass trajectory model attack angle is introduced to the calculation of drag coefficient and the differential equation of relationship between drag coefficient and resisting arm. The trajectory simulation is performed by Matlab It is concluded that the short resisting arm is the main reason for unexpected falling-down, and the countermeasures to avoid the unexpected falling-down is presented.