The invention discloses a short-distance nitrification/denitrification biomembrane nitrogen-removal reactor and a rapid membrane forming method. According to the invention, a transverse plate a, a vertical plate b, a vertical plate c and a transverse plate d are arranged in the reactor, the reactor is transversely divided into an upper part and a lower part by the transverse plate a , the lower part is a first anaerobic zone, the upper part is vertically divided into three parts by the vertical plate b and the vertical plate c, which are an aerobic zone, a transition zone and a second anaerobic zone; the lower end of the vertical plate b is connected with the transverse plate a, a water passing port is kept at the top of the upper end and the reactor, the upper end of the vertical plate c and the top of the reactor are connected in a sealing mode, the water passing port is kept at the lower end and the transverse plate a, the left end of the transverse plate d and the bottom of the vertical plate c are connected in the sealing mode; the right end and the wall of the reactor are connected in the sealing mode; and the transition zone is filled with palladium-copper supported active carbon catalyst particles. While the membrane is formed, water is introduced from bottom, the sewage passes through the first anaerobic zone, the aerobic zone, the transition zone and the second anaerobic zone in order, then is discharged from the upper part of the reactor, biomembranes at each area in the reactor are subjected to partitioning culture, the biomembrane having specific performance can be better formed, the membrane forming speed is rapid, and the nitrogen removal effect is good.