We comprehensively characterized the birefringence distribution of polarization-maintaining fibers (PMFs) in the fiber-optic gyroscopes using an enhanced Brillouin dynamic grating (BDG). This method enabled the quantitative analysis of the birefringence variations along the fiber, including those induced by temperature, axial strain, and transverse strain. Experimental results revealed that the birefringence coefficients of axial strain and temperature are 0.857×10-8/με and -4.7×10-7/℃ for the PMF coils, respectively. When PMFs are cross-wound in layered configuration within the fiber-optic gyroscopes, transverse-strain will significantly impact the birefringence distribution. These findings offer valuable technical guidance for the design and manufacturing of high-precision fiber-optic gyroscopes.