Abstract: The nonlinear effect of the lift coefficient of thesymmetrical airfoil near the 0^\circ angle of attack occurs at low Reynoldsnumber. It may be disadvantageous for control. The 3rd order Roe schemedeveloped by Rogers was used to solve the unsteady incompressibleNavier-Stokes equations, in the temporal direction the 2nd ordertwo-step method was accepted, to numerically simulate the flowfield laminarseparation vortex structure and the lift coefficient of the symmetricalairfoil SD8020 at low Reynolds number (Re=40\,000, 100\,000) at differentangle of attack. The correctness of the simulation was proved by comparingthe present CFD results with the experiment. Through the analysis and studyto the time-averaged result of the numerical simulations, the newtrailing-edge laminar separation bubble model for the airfoil near the0^\circ angle of attack was described in this paper, which is different fromthe classical laminar separation bubble model both in the inner structureand the development with the angle of attack. And this model was used tostudy and explain the mechanisms of nonlinear effect of the lift coefficientof the symmetrical airfoil at low Reynolds number at low angle of attack.