Based on the mathematical analysis, this paper used the panel data of 28 developed and major developing countries in 2001-2013, employed a Panel Smooth Threshold Regression model, empirically studied the nonlinear characteristics of the effect of public debt on economic growth. The results show that it has apparent "threshold effect" between public debt and economic growth, the "threshold" is about 150%, and it shows obvious "heterogeneity"in developed and developing countries. Considering the reasons of the effect of public debt on economic growth, its effect coefficient is influenced by the ratio of investment to GDP and the rate of inflation, but is more sensitive to the change of the former. In this paper, the conclusions further show that the present scale of public debt in China is relatively reasonable，it has positive impact on economic growth. But in recent years，the rapid growth and higher ratio of debt in local government needs great attention.

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