NO_x and PM_2.5 reduction were taken as indicators of emission reduction effect, and vehicle residual value cost, policy implementation cost and fuel cost saving were taken as social cost indicators. Based on social cost of per unit emission reduction (the ratio of social cost to emission reduction), cost-effectiveness analysis was conducted on the effect of the old motor vehicle replacement and subsidy policy in Beijing from June 2011 to December 2020. The results show that the cost-effectiveness of the policy is not monotonically decreasing over the period covered by the study. It shows phased and structural characteristics: the policy is most cost-effective in phasing out diesel trucks and least cost-effective in phasing out China III gasoline vehicles, and the cost of reducing per unit of NO_x and PM_2.5 differs by 14 times and 34 times, respectively; the cost-effectiveness of phasing out small buses tends to decline with the expansion of models to higher emission standards, and the emission reduction potential of small buses with China III and China IV Emission Standards has been limited and the cost-effectiveness is much lower than that under the early policy; the cost-effectiveness of phasing out heavy trucks is better and there are social cost savings in some periods. The paper suggests that the formulation of relevant follow-up policies should be based on cost-effectiveness. It should be cautious about including small passenger cars with higher emission standards in policy implementation, and focus more on the optimization of the policy design for accelerated phase-out of heavy diesel trucks.