We report a new mechanism for liquid crystal (LC)-based sensor system for trypsin detection. In this system, bovine serum albumin (BSA) was immobilized on gold grids as the enzymatic substrate. When the BSA-modified grid was filled with LC and immersed in the solution containing trypsin, the peptide bonds of BSA were hydrolyzed and peptide fragments were desorbed from the surface of gold grid, which disrupted the orientation of LC at the vicinity and resulted in a dark-to-bright transition of optical image of LCs. By using this mechanism, the limit of detection (LOD) of trypsin is 10 ng/mL, and it does not respond to thrombin and pepsin. Besides, the cleavage behavior on gold surfaces was directly visualized by the scanning photoelectron microscopy (SPEM), in particular for the chemical composition identification and element-resolved image. The loss of BSA fragments and the enhancement of Au photoelectron signal after trypsin cleavage were corresponding to the proposed mechanism of the LC-based sensor system. Because the signals reported by LC can be simply interpreted through the human naked-eye, it provides a simple method for fast-screening trypsin activity in aqueous solution.