In this paper noise-induced intermittency phenomenon in Hénon map is studied. Through numerical simulation and global analysis the mechanism of noise-induced intermittency in Hénon map is revealed. By using stochastic sensitivity function method to detect whether there will be collisions between confidence ellipse of periodic attractor and chaotic saddle, a method to estimate the critical strength of noise is proposed. Results show that the noise-induced intermittency in Hénon map is caused by interactions between cycle attractor and unstable manifold of chaotic saddle, whereas the stochastic sensitivity function method can estimate the critical noise strength in intermittency phenomenon accurately.