Fluid hydrodynamic fixed abrasive grinding (FHFAG) process is a promising finishing method for large optical elements which has been proposed in early research. However, liquid film characteristic remains unqualified given its complex working condition. In this research, the liquid film in the microchannel is used in a novel perspective. A fully coupled flow deformation model for fluid media subject to elasto-plastic rough surfaces is presented. This model predicts the concrete situation including dynamic pressure distribution and basic film thickness of the liquid flow in microchannel. A preliminary experiment has been done to verify the correctness. Our result shows that the liquid film can exist in the microchannel stably; there is a controllable mapping relation between the working parameters and the liquid film characteristics.