The invention provides a biological sensor for monitoring the sediment expansion height and a monitoring method. The biological sensor comprises an aerating device, a signal acquisition device, multiple micro-biological fuel cells and a support. The support comprises a horizontal separation plate. The cathode of each fuel cell is arranged on the horizontal separation plate, and the anode is arranged under the horizontal separation plate. The cathodes and the anodes are fixed to the support. The anodes are connected to the cathodes through wires. Resistors are arranged on the wires. At least one of N cathodes is arranged in overlying water of the sediment and all anodes are sunk into the sediment. N cathodes are in different heights. The signal acquisition device is connected with the micro-biological fuel cells. The aerating device is arranged in the overlying water and provides oxygen for the cathodes of the micro-biological fuel cells. According to the invention, the system and the method are highly sensitive, simple in structure, and low in building operation and maintenance cost, and are the method and the technology for effectively monitoring the expansion height of the sediment.