In this research, a polymorphic modulation method is proposed to suppress crosstalk by establishing a general formula adapted to different modulation depths; the optimal modulation method is determined by different modulation depths to eliminate the dead zone. According to the National Military test standard, the performance tests of the four-state and polymorphic modulations are carried out under modulation depths of π/2, 2π/3, 3π/4, 4π/5, 5π/6, and 7π/8. The dead zone thresholds corresponding to these depths are about 0.453, 0.741, 0.340, 0.623, 0.539, and 0.432 (°)/h. The zero offset stability corresponding to the use of polymorphism modulation is 0.0893, 0.0701, 0.0491, 0.0429, 0.0397, and 0.0365 (°)/h, respectively. The corresponding angular random walk coefficients are 0.0031, 0.0027, 0.0021, 0.0019, 0.0015, and 0.0012 (°)/h, respectively. Using the polymorphic modulation method derived from the general formula, the good performance of the fiber optic gyroscope can be maintained, and the obvious dead zone cannot be observed. Results show that the polymorphic square wave modulation has a significant advantage in eliminating the dead zone problem caused by crosstalk, which provides an important step for the realization of high-performance fiber optic gyroscopes.