Anaerobic ammonium oxidation (ANAMMOX) is known as the most effective process for nitrogen removal from organic wastewater with low ratio of carbon to nitrogen. But ANAMMOX population is difficult to enrich and sensitive to environment change, which results in a long start-up process. Thus, the tardy start-up of ANAMMOX process is a major issue for engineering application. In preliminary research, an upflow oxygen limitation biofilm reactor (UOLBR) was constructed to treat piggery wastewater with high ammonium (NH⁺₄-N) and low C/N ratio. After the 180-day start-up process, an excellent synchronous removal of chemical oxygen demand (COD) and total nitrogen (TN) was achieved with ANAMMOX as the dominant nitrogen removal pathway. Idled for 2 months, the UOLBR was restarted at 25 ℃ with a hydraulic retention time of 10 h and an effluent reflux ratio of 25∶1. Fed with manure-free piggery wastewater with the COD/TN ratio of 0.6-1.0, performance of the UOLBR during the restart-up process was evaluated. Operation process of the reactor was divided into 2 stages with a dissolved oxygen of 2.5-3.0 and 0.2-0.5 mg/L, respectively. The results showed that the UOLBR could recover from the idle state and a new steady state was reached within 53 days. Within the steady phase, the removal rates of COD, NH⁺₄-N, and TN in the reactor averaged 63.96%, 96.5% and 91.7%, with residues of about 1,7.0 and 16.4 mg/L in the effluent, respectively. Analysis of microbial community along with mass balance indicated that the recovered UOLBR was again characterized by ANAMMOX as the dominant approach for nitrogen removal from the wastewater. This work would be helpful for the start-up and management of ANAMMOX processes.