Disclosed is a flange surface microscale leakage on-line monitoring system, comprising a sealing sleeve (1), a connecting pipeline (2), a pressure transmitter (3), and an alarm (4), wherein the sealing sleeve (1) is used for sleeving the flange directly connected with a tank and forming a sealed space; the sealing sleeve (1) is provided with a hole which is connected to the pressure transmitter (3) via the connecting pipeline (2); the pressure transmitter (3) is used for detecting the pressure inside the sealed space formed by the sealing sleeve (1); and when the pressure is greater than the preset value, the pressure transmitter (3) sends signals to the alarm (4) for alarming. Unaffected by the external environment, the flange surface microscale leakage on-line monitoring system of the utility model is capable of rapidly monitoring the flange surface microscale leakage situation. The operators can be timely informed at the initial stage of the leakage, thereby avoiding the happening of greater leakage and minimizing the risk to the lowest level.