The invention discloses an auxiliary device for detecting linear change of a bridge and a detection method, and relates to an auxiliary device for detecting linear change of a bridge and a detection method, which aim at solving the problem of failure to detect due to damage of the monitoring points at the existing bridge deck by snow melting agents, snow cleaning vehicles, artificial factors and other factors. The auxiliary device comprises an embedding device, a detecting device and a sealing device, wherein the embedding device is a hexagonal prism of which the upper end is provided with a blind hole; the inner wall of the blind hole is provided with threads; the embedding device is embedded into the bridge deck, and is vertical to the bridge deck; the lower end of the embedding device is connected with a main beam of the bridge, and the upper end of the embedding device is 2 to 3cm lower than the bridge deck; the detecting device comprises a cylinder and a spiral column, one end ofthe spiral column is fixedly connected with one end surface of the cylinder, the center line of the cylinder is coincided with the center line of the spiral column, and the spiral column is matched with the blind hole of the embedding device in use during detection; the sealing device is of a bolt structure, and is matched with the blind hole of the embedding device in use. The auxiliary device isapplied to the field of bridge detection.