A method for improving the evaluation of cell deformability is proposed. Cell deformability can be evaluated by the ratio between the cell velocities before and after entering the narrow throat of a microchannel. However, the flow-in velocity, the cell velocity before entering the throat, varies with respect to the location of the cell because of the laminar flow inside the microchannel. To compensate such velocity variation caused by different locations, we obtain a mean velocity by normalizing the velocity profile, and utilize this mean velocity for evaluating cell deformability. Experimental study on human red blood cells is performed. By implementing a high-speed vision system, we acquire the information of the location and velocity of every single cell through the narrow throat. The acquired data are processed by the proposed method. We compare both the results with and without the normalization, and find that the the proposed method effectively improve the consistency and accuracy of the evaluation.