In the traditional ambient occlusion algorithm for microstructures, the determination of occlusion is a time-consuming task. To improve the rendering speed, the paper proposes a screen-based ambient occlusion method. By using off-screen technique, only the visible points are used to determine the occlusion. Using the depth texture of the whole scene, the size of ambient occlusion can be quickly computed by the maximum of the block angle. The whole scene can be rendered using the deferred shading technology. Experimental results show that our method is more efficient than the available methods on computing the ambient occlusion. It does not need any pre-processing, and the time used for ambient occlusion determination is less than other methods.