In view of the significance, complexity, and difficulty of micromixing in viscous system, coaxial mixers with wide adaptability 'to viscosity are first combined with micromixing. The effects of feeding position, rotation mode, and impeller type on the micromixing performance of coaxial mixers are investigated numerically with the standard engulfment model and eddy dissipation concept model, and a dimensionless estimating correlation of segregation index X-s is established based on the Buckingham pi theorem. The results show that feeding near the discharge zone of the impeller favors the micromixing and Rushton turbine impeller (RT) is better qualified as the impeller near feeding point compared to six-straight-blade turbine impeller (SBT) and six-pitched (45 degrees)-blade turbine impeller (PBT). At the same specific power consumption, the coaxial mixer has better micromixing performance with PBT + RT as inner impeller than with SBT + RT, and the difference of segregation index resulting from the rotation mode is small. Moreover, the proposed correlation is validated to be suitable for qualitative evaluation of micromixing performance of coaxial mixer consisting of inner impeller combining PBT and RT and outer anchor.