Population aging not only affects the stability of macro-economy, but also affects the effectiveness of fiscal policy. In this paper，we consider a new Keynesian DSGE model with heterogeneous agent as workers and retirees, and investigate how the aging affects the effectiveness of government spending shocks, labor income tax threshold adjustment shocks, labor income tax progressiveness adjustment shocks and government public investment shocks. The result shows that aging expanded the squeezing effect of governmental expenditure and public investment on consumption, decreased the boosting effects of four kinds of positive fiscal policies on output, consumption and employment, and strengthened the raising effect on inflation and wage expansion. The extended retirement age does not produce squeezing effect on private consumption but is conducive to employment, output and consumption growth and is helpful to stabilizing inflation and wage growth, compared with fiscal policy tool, the total social welfare loss resulting from extended retirement age is smaller. The aging decreased the effectiveness of positive fiscal policy, then, for the purpose of stabilizing macroeconomics and decreasing social welfare loss, the policy for extending the retirement age is meaningful.