An open land combined foundation pit excavation monitoring and early warning system suitable for a high slope and river channel double steps comprises an unmanned aerial vehicle, a soil body displacement monitoring device and a high slope sliding monitoring device, the soil body displacement monitoring device is located in a river channel, and the high slope sliding monitoring device is located in the high slope; the soil displacement monitoring device comprises a steel shell, a center rod piece, a fixing bolt, a transmission antenna, a laser lens, an anchoring device, a threaded opening, a limiting baffle, a rotating shaft, a piston, a hydraulic five-way valve, an anchoring horizontal pile, a horizontal displacement monitoring device, a power source, an ampere meter, data transmission equipment and a drill bit; and the high slope sliding monitoring device comprises a positioning rod, a sliding surface monitoring unit, a high-strength spring, a guide rod, a steel ejector pin, a pressure position sensor and an upper transmission unit; and an infrared thermal imaging camera and a laser range finder are installed on the unmanned aerial vehicle. The invention further provides an operation method of the monitoring and early warning system. The system has an intelligent early warning function, and the safety of foundation pit engineering construction can be guaranteed.