Instability of high solid-state anaerobic digestion often happened during the longterm operation of reactor. To investigate underlying methanism of this problem, the characteristics of semicontinuous anaerobic digestion for rice straw were studied with three organic loading rates (OLR) in a horizontal reactor, when being co-inoculated with ruminal contents and anaerobic sludge. The results indicated that the highest volume biogas production reached 1.04L/(L·d) in the reactor. The average CH4 productivity reached 280.90mL/g VS with 54.39% CH4 volume content at the OLR of 2.26g/(L·d), which was up to 80.29% of the theoretical value of rice straw. The highest digestion rates of hemicellulose and cellulose were 49.71% and 31.25%, respectively. And the cellulase activities were increased obviously in the horizontal reactor, which was indispensable to the degradation of cellulose. At the OLR of 2.47g/(L·d), the average concentration of ammonium nitrogen reached 1082.63mg/L. Especially the highest concentration reached 1196.37mg/L after 192d of operation. It was indicated that the methanogens suffered different influences from ammonium nitrogen using quantitative realtime PCR(Q-PCR). Specifically, the hydrogenotrophic Methanobacteriales was decreased to 1.04×106 copies/g from 1.70×109copies/g dominated in the solid residues and CH4 productivity was decreased to 256.54mL/g for being inhibited significantly. In contrast, the acetotrophic Methanosarcinales was increased to 9.44×106 copies/g from 7.89×106 copies/g. At the same time, the average propionate concentration reached 253.32mg/L in the anaerobic reactor. Thus the study revealed the adaptive shifts of underlying methanogenic communities by ammonium nitrogen inhibition during the high solidstate digestion of rice straw.