Combined biological filter

The utility model discloses a combined biological filter which comprises a filter body, a back-flushing water pipe is arranged at the bottom of the inner side of the filter body, a back-flushing air pipe is arranged at the top of the back-flushing water pipe, the bottom of the inner side of the filter body is filled with a supporting layer, an air pipe is arranged at the position, located at the top of the back-flushing air pipe, of the inner side of the filter body, and the air pipe is communicated with the back-flushing air pipe. A water outlet is formed in the edge of the top of the inner side of the pool body, and a winding box is fixedly arranged at one end of the top of the pool body; after the pool body is used, the covering cloth in the winding box can be pulled out, the pool body is covered with the covering cloth, the pool body is protected through the covering cloth, it is prevented that more impurities and the like fall into the pool body when the pool body is idle, and use of the pool body is aff