The invention discloses a fixing device and a method for accurately mounting a valve seat, and belongs to the technical field of valve seat mounting. The fixing device comprises a plurality of groupsof first fixing pieces and second fixing pieces which are matched with each other; each first fixing piece sequentially comprises a first threaded connection part, an extension shaft and a first operating part which are connected with a threaded hole of a valve body; a first connecting platform is arranged at one end of the first threaded connecting part which is connected with the extension shaft; each second fixing piece comprises a second threaded connecting part and a second operating part which are simultaneously connected with the threaded hole of the valve body and a threaded hole of the valve seat; each second fixing piece is provided with a center through hole matched with the extension shaft; a second connecting platform matched with the first connecting platform is arranged at one end of the second threaded connecting part which is far away from the second operating part; and the outer diameter of the first threaded connecting part is the same as that of the second threadedconnecting part. The large-caliber valve seat can be effectively and accurately mounted on the valve body in a vertical state, so that the sealing performance of the valve seat and the high reliability of the valve seat are ensured.