Updating rules in the flow tables of SDN switches are complex and time-consuming. Therefore, we propose a cache-based scheme (named FlowShadow) to improve the packet processing performance and keep continuous operating while updating rules in the flow tables. FlowShadow caches the microflows in the hash table to build a fast path for packet processing. By leveraging the Action Table, FlowShadow achieves update consistency and good update performance. In order to examine the reliability, validity, utility and scalability of FlowShadow, we implement FlowShadow on the Open VSwitch and conduct numerous experiments with different settings to measure the performance of FlowShadow. The experimental results demonstrate that FlowShadow achieves a lookup speed of 75 million packets per second on a commodity PC under the real backbone traces; the system with FlowShadow speeds up 3.4× times of the original Open VSwitch.