The invention relates to a gravel pack filling monitoring system and method. The gravel pack filling monitoring system comprises a magnetic particle container, a gravel pack container, a gravel pack mixer, a sand-carrying fluid container, a sand mixing truck, a high-pressure pumping set, a data acquisition and processing center, a ground magnetometer, a magnetic particle conveying pipeline, a gravel pack conveying pipeline, a evenly-mixing gravel pack conveying pipeline, a sand-carrying fluid conveying pipeline, a first sand-mixing fluid conveying pipeline, a second sand-mixing fluid conveying pipeline and a ground magnetometer and data acquisition and processing center communication pipeline, wherein the ground magnetometer monitors magnetic parameter changing from the ground, the data acquisition and processing center is connected with the data acquisition and processing center communication pipeline through the ground magnetometer, and the data acquisition and processing center collects and stores the measured information of the ground magnetometer and calculates and displays the gravel pack distribution state. The gravel pack filling monitoring system can track the transporting and distributing state of gravel pack in a sand-prevention well section from the ground, the well and/or an adjacent well during gravel pack filling and can achieve repeated detection on gravel pack distribution after production of the sand-prevention well.