The invention discloses a device for monitoring coal body transverse deformation on line and a monitoring method thereof, and belongs to the fields of coal mine impact earth pressure control and mine rock deformation monitoring. The device comprises a data processing unit arranged on ground, a data transmission unit arranged on a working surface and at least one measuring unit arranged in a drilling hole. The measuring unit comprises a hydraulic type transverse displacement determinator, a fixed clamping ring and a pushing rod. The fixed clamping ring is of a hollow ring-shaped structure and comprises a ring surface and a cavity. A line port is arranged on the part, close to the outer side, of the ring surface. At least one clamping nail and guiding groove matched with the clamping nail are distributed on the ring surface. The clamping nail is capable of sliding clockwise in the guiding groove. A spine part of the clamping nail extends out from the ring surface. Steps are arranged in the clockwise sliding direction of the clamping nail, and a certain included angle is formed between the bottom layer of the steps and the clamping nail. According to the invention, a plurality of devices can be distributed in one drilling hole, one-hole multi-point monitoring is realized, mutual interference among the monitoring points is also avoided, and the data is accurate and effective.