A design method of sliding mode approaching law was applied in attitude control of vertical water-exit of submarine-to-air missile. Firstly, quaternion was used for submarine-to-air missile of vertical water-exit to establish a unified equation form of underwater and in air motion in missile's body coordinate system, then the small perturbation linearization state space model was adopted in pitch channel to design attitude control law based on sliding mode approaching law, finally the 6-DOF trajectory simulation model in Simulink was established and simulated. The result indicates that the unified equation form of underwater and in air motion using quaternion is efficient, and attitude control law based on sliding mode approaching law is practicable in vertical water-exit control of submarine-to air missile.