An earthquake monitoring system includes a first monitor and a computer system. The first monitor is configured to photograph at least one reference point disposed on a first floor slab of a building to generate a first initial image and a first image stream. The first monitor is disposed on a second floor slab of the building. The computer system is configured to process the first initial image and the first image stream, to obtain a first interlayer relative displacement vector between the first floor and the second floor according to the first image stream and a rotation angle of the first monitor, and output a warning signal according to the first interlayer relative displacement vector.