The invention provides a real-time on-line monitoring system for urban excavation safety construction. A monitoring device is installed on an excavator, the monitoring device comprises a main controlchip, a methane detection module, an alarm module, a positioning module, a communication unit and a digital compass, the methane detection module is used for detecting methane and detecting the methane concentration, the positioning module is used for detecting the current position of the excavator in real time, the main control chip is used for sending position information obtained by the positioning module to a display terminal in real time, a fuel gas pipe network circuit diagram of the current position area of the excavator is pre-stored in the display terminal, and the display terminal isused for carrying out three-dimensional display on the relative positions of the excavator and the fuel gas pipe network circuit diagram. According to the monitoring system, precautions and real-timemonitoring of fuel gas leakage are achieved in the excavation process so that the gas leakage risk in the excavation process can be reduced, and the loss caused by gas leakage can be reduced.