The flow chamber is widely used to study the cell's mechanics behavior, especially the adhesion property. Its length and width are far larger than the height. The paper pointed out that the formula people used to calculate the shear stress was appropriate when Re was small, then gave the rational size of the flow chamber. Through an example, the paper got a result that the shear stress formula was suitable only in the middle region of the flow chamber. It provided the theoretical basis for the rational application of the flow chamber and the analysis of the cell's mechanics behavior.