Dynamic stall of NACA airfoils of three kinds of different relative thickness undergoing pitching motion at low Mach number was numerically investigated. Dual time-step approach in conjunction with lower-upper symmetric-gauss-seidel(LU-SGS) implicit method were utilized to solve the unsteady navier-stokes (NS) equations in noninertial system, and Roe scheme was brought in for space discretization, incombined with stiff moving mesh generation technology. Fully turbulent solution was adopted, by using baldwin-lomax(BL) model. The computed load hysteresis loops indicate good agreement with the experiment results,which confirms that the numerical method is effective. Through comparing and analyzing different flow fields, it is simultaneously founded that the formation and development of stall vortex passing airfoil is obvious distinct for different-thickness airfoils during light stall and deep stall.