Soft shadow generation is a challenging problem in realistic rendering. Previous methods using shadow map or shadow volume work well for point light sources but are difficult to be extended to area lights. This paper presents a new method for fast soft shadow generation under dynamic area light sources. Our algorithm encodes the depth distribution of the scene into a coarse depth grid in a preliminary pass from the light point of view. In the second pass, the scene is rendered from the camera viewpoint to capture the frontmost layer. During deferred shading, the area light is sampled and the irradiance of each shaded pixel is accumulated along the ray. Experimental results demonstrate high quality soft shadows with interactive performance for dynamic scenes and lighting.