The invention discloses a bridge flexibility monitoring system. A horizontal communicating pipe 1 is laid along the transverse direction of a bridge beam. A vertical communicating pipe 2 is longitudinally laid at each observation point. Each vertical communicating pipe is communicated with the horizontal communicating pipe. A microwave float 3 is placed on the liquid level in the vertical communicating pipe. A microwave radar 4 is mounted on the top of each vertical communicating pipe. A liquid level height change amount is measured through measuring the phase different change amount of an RF carrier between the microwave radar and the microwave float, thereby measuring the flexibility value at each observation point. The bridge flexibility monitoring system can realize high-precision automatic measurement on bridge flexibility.