Back door reinforcer and back door

The back door reinforcing piece comprises a bottom plate, a lower side plate with the front edge connected with the lower edge of the bottom plate and a lower flanging with the upper edge connected with the rear edge of the lower side plate, the lower flanging is provided with a long-strip-shaped glue groove, the long-strip-shaped glue groove is concave forwards, and the length of the long-strip-shaped glue groove extends in the left-right direction. A strip-shaped reinforcing rib and a corner reinforcing rib are further included, the corner reinforcing rib is arranged at the corner between the bottom plate and the lower side plate, the strip-shaped reinforcing rib comprises a first section and a second section, the first section extends forwards from the rear edge of the lower side plate to the front edge of the lower side plate, the second section extends upwards from the lower edge of the bottom plate, and the lower end of the second section is connected with the front end of the first section. The back do