Abstract: The temporal and spatial coherence which is an important characteristic of unsteady flows plays an essential role in visualizing time-varying fields.This paper presents an approach for high-efficiency volume rendering of unstructured time-varying flows using the temporal and spatial coherence on the framework of the hardware-based ray casting algorithm（HRC）.Firstly,a method is provided to analyze the temporal coherence of both the cell and the vertex data on unstructured grids.Then the cell and the vertex data temporal tables are built to achieve a lower time cost during rendering.Secondly,a novel texture structure is designed to separate the vertex data from the cell data,and a smart gradient matrix is used to reduce the pressure of GPU memory.The scheme of data management can effectively avoid rendering stalls and lead to a compact and efficient storage.The experiments demonstrate that our approach not only gains a much higher efficiency than the existing method,but also achieves a lower space cost,allowing rendering time-varying data on a larger mesh scale.