The parallel-plate flow chamber(PPFC)with rectangular shape,of which the height is far smaller than its own length and width,is one of the main apparatus for in vitro studies of the mechanical behavior of cultured en-dothelial cells undergoing shear stress.The PPFCs are usually utilized to study the cells'mechanical behavior under steady flow condition.Considering that the Reynolds numbers of the flows in the usually used PPFCs are small,a perturbation solution of laminar pulsatile flows in the PPFCs is presented using Reynolds number as perturbation parameter.The0-order,first-order,second-order approximate calculation formulas of the velocities in the PPFCs and the shear stress at the bottom of the PPFCs are given in this paper.The calculated numeri-cal results and the comparison between the perturbation solution and the Fourier solution demonstrate that,the approximate formulas are highly precise for various commonly used flow parameters and geometrical dimensions of the PPFCs.In particular,the0-order approximate formula,the quasi-steady formula,is much more appropri-ate to the pulsatile flow with higher Reynolds number,which provides a practical method for the calculation of the shear stress in the PPFC under pulsatile flow condition.