A generally realized phenomenon in the suction experiment of cells is that if the aspiration pressure is larger than a certain threshold, cell flows continuously into the pipette. The point of the threshold aspiration pressure at which the cell can still be held in a stable equilibrium is called the critical point of aspiration. Here we represent a theoretical analysis of the equilibrium behavior and stability of cell by liquid drop model. In the method of analysis, the areal change due to a small movement deltaL of the portion of the membrane in pipette is given to the first approximation. The threshold pressure and the critical point are shown as simple formulas of the model parameters and inner relative radius of pipette. The results derived from formulas are consistent with rigorous ones by numerical computation in the approximate range.