The invention belongs to the technical field of power station boiler acoustical monitoring and particularly relates to a power station boiler heat expansion monitoring system and a measuring method based on the acoustical principle. The system mainly consists of a sound wave generator, a sound wave receiver, a sound wave guide tube, a signal conditioner, a data wire, an input/output device, a wiring box, a collecting card and a computer host, wherein the sound wave generator is arranged at the measuring point arranged on a boiler. The sound wave generator is controlled to send out sound signals through host program, the signals are input into the host through a transmission wire after being received by the sound wave receiver, through calculation and analysis processing, the signals are displayed on the host in chart and simulated diagram modes, clearness and intuition are realized, and operators can conveniently know the expansion condition of boiler equipment in real time. The work intensity of the operators is lightened, the risk is reduced, and the operation level of a thermal power unit is improved.