The invention discloses an online monitoring device and method of subway contact net support looseness. The device comprises an optical fiber sensing dynamic analyzer, a transmission optical fiber, and a contact net support which is arranged between each contact net and a tunnel top. One end of the contact net support is provided with a connecting piece, one end of the connecting piece is fixedly connected to the end part of the contact net support, and the other end is fixedly connected to the tunnel top. Optical fiber strain sensors are fixed to the connecting piece, and multiple optical fiber strain sensors are connected in series through the transmission optical fiber and then are connected to a dynamic optical fiber analyzer to form a loop. According to the invention, the remote online real-time monitoring of the fastening condition of the contact net can be realized, a lot of time and manpower consumed by manual inspection is saved, a rich data support is provided for the maintenance and reinforcement of a contact net fastener, positioning maintenance according to needs is achieved truly, and the practicability is high.