Monitoring and early warning systems are an efficient way to avoid and mitigate the significant loss of properties and people’s lives caused by water inrush via the coal seam floor, a frequent problem in north China’s deeper coal mines. This paper discusses the basic technical problems, such as the monitoring conditions, indexes and technical system of monitoring and early warning of water inrush in coal seam floor, and selection of monitoring positions. Double criteria for water inrush monitoring and early warning were established based on three factors: crack instability extension, water temperature, and water pressure. Accordingly, a new monitoring and early warning system of the water inrush in coal seam floor was developed using a fiber Bragg grating sensor. This was applied at coal face 2113 of the Chengjiao mine, Henan Province, which lowered the water inrush probability, and so no early warning was necessary. The rationality and effectiveness of this technique were thus verified.