The invention discloses an auxiliary device and method used for laying tunnel waterproofing plates, and the auxiliary device comprises an arch-shaped support, a guide rail, a plurality of guide rolls and a traveling trolley, wherein, the guide rail is arranged along the outer outline of the arch-shaped support; the guide rolls are arranged at the inner side of the guide rail horizontally and parallelly; the guide rolls are supported by the arch-shaped support; the traveling trolley comprises a traveling wheel, a clamp component, a guide component and a trolley support, wherein, the trolley support comprises a first support group and a second support group; the upper part of the first support group is provided with the guide component which is a steering roll; the other end of a support rod of the steering roll is connected with the first support group; and the clamp component is arranged on a connection rod, and comprises two clamp rolls rotationally connected on the connection rod; and a clearance is formed between the two clamp rolls. The auxiliary device and method provided by the invention have the advantages of being beneficial to laying, lowering the construction difficulty and labor intensity, improving the working efficiency and ensuring the laying quality.