Numerical simulations were performed to study the affect of unsteady wake on leading edge film cooling.The calculated film cooling effectiveness was compared with the experimental results.The unsteady numerical simulation shows that the stagnation point moving to the cooling flow slit causes the cooling flow to diverge when the unsteady wake swept through the film-cooling slit.The divergence of the cooling flow results in sharply decreasing of the film cooling effectiveness downstream the film-cooling slit.