Diet is the most direct way to affect the rumen microbial community. The objective of this study was to examine the effects of different diets on ruminal methanogen composition in cashmere goats. Twenty-four cashmere goats were randomly divided into four groups that were fed alfalfa hay (AH), alfalfa hay plus concentrate (AHC), corn stalks (CS), and corn stalks plus concentrate (CSC). The concentration of ammonia-N for the group fed AHC was substantially higher than in the other groups. The concentrations of total VFAs, acetate, and propionate in AH and AHC groups were higher than in the CS and CSC groups. The abundance of archaea was markedly different among different groups based on a high-throughput sequencing method. The abundance of Candidatus_Methanomethylophilus was 21.28% in the AH group, which was higher than the other groups. Unclassified_c_Thermoplasmata was the dominant methanogen in the AH and AHC groups, with abundances of 48.09% and 44.97%, respectively; Methanobrevibacter was the dominant methanogen in the CS and CSC groups, with abundances of 41.22% and 20.73%, respectively. A positive correlation was identified between pH and Methanosphaera; a negative relationship was observed between acetate and Unclassified_o_Thermoplasmatales. In conclusion, the methanogen communities in cashmere goats varied on different diets. In the alfalfa hay-based diets, the Methanomassiliicoccales-affiliated groups were the dominant methanogens in the rumen of the goats. However, when fed the corn stalk-based diets, Methanobrevibacter was the dominant methanogen in the rumen fluid. The results were related to dietary composition, especially crude protein content.