Abstract: In order to solve the key scientific problems in control difficulty of thermal-fluid-structure coupling deformation in mold micro assembly process, a theoretical model was established to describe mechanisms of thermal-fluid-structure coupling deformation in polymer in-mold micro assembly molding process on the basis of the influence of environmental impact on secondary molding viscoelastic melt filling flow. A finite element numerical simulation was also conducted to investigate the effect of secondary molding melt viscosity on the thermal-fluid-structure coupling deformation during the mold micro assembly process. The results indicated that the driving force of the micro shaft deformation induced by the thermal-fluid-structure coupling effect was derived from the thermal-fluid-structure coupling pressure and viscous friction drag shear stress on the micro assembly surface. However, the elastic normal stress of secondary molding viscoelastic melt filling flow showed a restraining effect on the coupling deformation. The thermal-fluid-structure coupling load and micro shaft coupling deformation increased with increasing secondary molding melt viscosity, which reduced the viscosity of secondary molding melt. This is beneficial to the improvement in processing accuracy of the micro assembly.