Understanding the strain localization process and the influence of accompanying microstructures during ductile deformationon the second phase of main phase mineral microstructure evolution in multiphase mylonite has presented a challenge in microstructural geology research. Previous studies have shown that grain boundary sliding is a potential mechanism for achieving multiphase mineral mixing to form multiphase aggregation in mylonite.In fact, natural mylonite is normally composed of multiphase minerals. Generally, in the multiphase mylonite, the second phase exerts a zener resistance at the boundary of the matrix phase grains, which restrains the migration rate of the grain boundaries in the matrix phase. This can destroy the dynamic equilibrium process of the matrix phase grains, and lead to the matrix phase grains below the grain size corresponding to the paleopiezometer.Furthermore, after the matrix phasegrain sizedecreases, the overall surface area of the matrix phase increases,whichwill contribute to the diffusion exchange process. In addition, due to the contribution of diffusion creep and the lowerefficiency of dislocation creep, the deformation mechanismchanges from grain size insensitive creep mechanism (GSI) to grain size sensitive creep mechanism (GSS). Moreover, the second phase in the multiphase mylonite has an effect on strain localization initiation, which probably changes the material strength, the rock deformation process, as well as rheological behavior.In this paper, based on the summary of previous research results,granitic mylonite from the Qinling Group is selected for the quantitative study. The preliminary results show that grain size of quartz and strength of its CPO is significantly reduced with increased mica content and the degree of mixing of the mineral phases within the granitic mylonite. Thus, we conclude that the microstructure of the matrix phase is gradually controlled by the second phase during mylonitic deformation in the natural mylonite.