The invention discloses a microseismic monitoring-based coal mine floor damage depth determining method and belongs to the coal mine safe production technical field. The main objective of the invention is to determine damage depth by using microseismic monitoring. The method includes the following steps that: the average elevation of a working face coal seam floor is determined; microseismic geophones are arranged; filtering, microseismic event positioning and related seismic source parameter determination are carried out; floor microseismic events are analyzed, the number of the floor microseismic events and a distribution curve of energy along depth are determined; and floor damage depth can be determined according to the average elevation of the working face coal seam floor, the number of the floor microseismic events and the distribution curve of energy along depth. With the method of the invention adopted, dynamic monitoring of the floor damage depth is realized; the determination of the development trend of the floor damage depth can be facilitated; early warning against floor water inrush accidents can be carried out; and a horizon where floor damage appears can be determined, so that a basis can be provided for floor water prevention and control work.