To further improve the precision of rotational strap-down inertial navigation system based on fiber optic gyro, an appropriate digital position stabilization loop is designed. The requirement of strap-down inertial navigation system to stabilization loop is analyzed. Simple and practical models are built and test methods of rotational inertia and friction torque in these models are given. The proportional integral double-integral(PII²) controller and strategy for its parameters are studied. The simulation results show that the PII² controller is more effective to decrease steady-state angle error than the PID controller in this stabilization loop based on the fiber optic gyro. The experimental results show that this stabilization loop’s dynamic precision is high and meets the design requirements.