The three-loop autopilot is employed by spinning missiles as well as by many high-performance command or homing guidance missiles currently because it performs well in stabilizing airframe and implementing guidance commands. However, for spinning missiles, the closed-loop system may be dynamically unstable in the form of a divergent coning motion due to the existence of cross-coupling effects. And the stability criteria of the autopilot applicable to the nonspinning missile are no longer valid in the event of the spinning. To address this issue, the structure of a three-loop autopilot of spinning missiles is introduced in this study, for which the sufficient and necessary condition of coning motion stability is analytically derived from the equations in the form of complex summation. The stability criteria are further illustrated by numerical simulation. It is noticed that spinning shrinks the stable region of the design parameters significantly. And the higher the spinning rate, the smaller the stable region becomes.