The dead-zone attached with all-digital close-loop Fiber Optic Gyro (FOG) which employs square wave bias modulation and step wave feedback modulation was researched. The dependence between the height of step wave and open-loop output of the FOG was obtained with a correlation coefficient equaling 0.97,and an open-loop detection model taking this correlation into account was established. Based on the equivalence of digital step wave height set up by fixed step or feedback step,the above open-loop model was modified to close-loop model. The close-loop model was analyzed by theory and simulation. The simulation dead-zone range was compatible with the experimental one with an error of 6.7%. The results show that the linear correlation between the height of step wave and the output is one of the factors of dead zone.