Abstract: This article primarily addressed the theoretical framework and the application background of optical flow estimation technology, and described the state-of-the-art popular optical flow algorithms and the evolving mathematical models. The technical emphasis and difficulties involved in the optimization of classical optical flow energy model were analyzed from 5 different aspects: edge preserving smoothing, vary illumination tackling, large displacement optical flow estimating, outlier removing and real-time computing. The existing proposed corresponding solutions were introduced in detail and the substantial research directions in the future were prospected. This article conclude that large displacement optical flow estimating, vary illumination tackling, and real-time computing will become the attractive topics on optical flow in the future.