The invention relates to a step-feed biological nitrogen removal and sludge decrement coupling bioreactor and a process thereof, belonging to the field of sewage treatment and aiming to overcome the technical defect that the sludge output is large, and the total nitrogen removal effect of sewage with a low carbon-nitrogen ratio is poor. The bioreactor sequentially comprises a biological reaction area and a secondary clarifier, wherein a biological reaction area sequentially comprises an aerobic area, an anoxic area, an aerobic area, an anoxic area, an aerobic area, an anoxic area, an aerobic area and an anoxic area, and the volume ratio of each aerobic area to each anoxic area is (1.5:1)-(2:1); a porous carrier is arranged inside the biological reaction area; the carrier filling volume ratio of the aerobic areas is 40-60 percent, and the carrier filling volume ratio of the anoxic area is 90-100 percent; and all the areas are respectively provided with a baffle plate. The bioreactor is divided into three water inlet stages, the secondary clarifier is provided with a water outlet pipe and a sludge discharge pipe, and backflow sludge is conveyed to the aerobic areas O-I through a pump. The TN removal ratio of the bioreactor is 75.3 percent, the COD removal ratio is 94.3 percent, and the sludge productivity is 0.102kgMLSS / kgCOD.