In order to solve the problem of rendering Depth Of Field (DOF) in Virtual Reality (VR) system, an improved post-processing algorithm of DOF based on Graphics Processing Unit (GPU) was proposed. First, the scene at full resolution was rendered to an offscreen buffer, and the linear depth of each pixel was output to the alpha channel of the buffer. Then, the fully-rendered scene was downsampled into 1/16 of the original size. Next, the downsampled scene was blurred by running the image through two passes of a separable Gaussian filter, and stored as a texture. Finally, the two textures were blended according to the linear depth of each pixel in the size of Circle of Confusion (CoC). The algorithm was applied in a virtual roaming scene and a good simulation of real-time DOF effect was achieved. The experimental results show that the algorithm has improved some problems in the traditional post-processing filter algorithm, such as poor continuity and brightness diffusion, and it meets the demand of real-time interaction in VR.