Anammox (anaerobic ammonium oxidation) is a promising technology to remove nitrogen from wastewater, but the long start-up time limits its practical application. This study used an up-flow anaerobic sludge bed (UASB) reactor, selected anaerobic granular sludge as inoculating sludge, and maintained pH (7.8-8.2), temperature (30-35°C), and DO (≤0.5 mg L^-1) to start an anammox process. The initial start-up process occurred in three stages: cell lysis phase, activity lag phase, activity elevation phase. In the cell lysis (1-2 d) and activity lag (3-4 d) phases, denitrification was the main reaction and TN removal rates were 36.72% and 52.34%, respectively. In the activity elevation phase (5-30 d), denitrification weakened, anammox activity strengthened, NH₄⁺-N removal rate was increased greatly, NO₂^--N removal rate took on the trend of increasing at first and decreasing later, and the average removal rate of TN rose to 74.33% and reached the maximum rate of 81.91%. The variations of sludge bed in the cell lysis phase and activity lag phase were not obvious; but in the activity elevation phase, the sludge bed gradually varied from the bottom to the top, and the color of the sludge changed from black to brown, and a little sludge was red.