A framework for online automatically discovering fine operational rules of power system is proposed， based on online automatic discovery of critical flowgates， fine operational rules are discovered online through automatic learning method. An online automatic discovery system of power system fine operational rules is proposed， and functions of each core module of the proposed system is also proposed， such as online automatic discovery of critical flowgates， establishment of initial operational rules， sample simulation， feature selection， establishment of fine operational rules and so on. The proposed system is applied on a 4-generation 2-area test system and a provincial power system of China， and results show that the fine operational rules which have better accuracy and interpretability are more suitable for power system operation and the transfer capacity of the critical flowgates can be utilized more efficiently， at the same time， the fine operational rules can be discovered in 15 minutes， meeting the requirement of online application.