Cells have to undergo many changes in osmotic pressure during their long-term preservation, and will have injuries before they return to their normal states. Mechanics of a cell with deformation, either small or large, is usually used to describe the change of the cell quantitatively. However, there are few reports on the deformation of cells subjected to the change of osmotic pressures during preservation. Here, we report our study of the elasticity of the human red blood cells under osmotic pressures using optical tweezers. We find that the deformation characteristics of erythrocytes are strongly dependent on the osmotic pressure. We also find the RBCs will become stiff with increasing osmotic pressure, suggesting a potential reason for membrane injury during preservation.