Roll-isolation is an effective way for spinning vehicle to greatly reduce the roll gyro range of strapdown Inertial Navigation System (SINS) and increase the accuracy of inertial navigation. However, during a recent flight test, the roll-isolated control system failure was observed under a large pitch angle (70? 6 h 685? ), which introduces a sharply increase in the roll angular velocity, the saturation of roll gyro and the inertial navigation failure. To address this issue, the governing equation of the roll-isolated system is derived with the consideration of various disturbance factors. The control failure is reproduced by numerical simulation. And the results show that the pitch and yaw angular velocity can cause a dramatic increase in roll rate under the large pitch angle, resulting in the roll-isolated control failure. Meanwhile, an improved roll-isolated control system is devel-oped using PI controller, which is verified by mathematical simulation.