For studying the steady scanning mechanism of a certain type of rotary self-stabilization smart submunition, according to the characteristics of asymmetric structure and throwing conditions, the ballistic motion model was established to easily quantify submunition's dynamic unbalance. By setting the non-axisymmetric mass in the projectile body, the 6 DOF motion differential equations of the submunition were derived, and the numerical solutions of the equations were obtained. Numerical solution results showed that the direct cause of the steady scanning motion of the projectile was the dynamic unbalance which caused by the non-axisymmetric distribution in the rotary projectile body. The differential equation of the simplified 6 DOF motion could accurately simulate the steady scanning motion of the submunition, and provided theoretical basis for the overall design of the smart submunition.