Under the framework of endogenous economic growth, infrastructure variable is introduced. The theoretical model prediction results show that the relationship between infrastructure and economic growth is uncertain, and may be linear or nonlinear. The empirical analysis of B &R countries shows that the promotion effect of infrastructure on economic growth has not only a single threshold effect, but also a marginal efficiency decline effect. The impact of infrastructure on economic growth depends on per capita income and political environment. When per capita income exceeds the threshold, improvement of infrastructure can promote economic growth; when the political environment exceeds the threshold, the improvement of infrastructure contributes more to economic growth.