The unbridled expansion of moso bamboo (Phyllostachys edulis) occurs throughout the world and has a series of consequences. However, the effect of bamboo expansion on arbuscular mycorrhizal fungi (AMF) is still poorly understood. We assessed the changes in the AMF community during bamboo expansion into Japanese cedar (Cryptomeria japonica) forests by analyzing AMF in three forest types—Japanese cedar (JC), bamboo-cedar mixed (BC) and moso bamboo (MB)—using 454 pyrosequencing technology. We found that the AMF community composition differed significantly among forest types. The relative abundance of Glomerales decreased from 74.0% in JC to 61.8% in BC and 42.5% in MB, whereas the relative abundance of Rhizophagus increased from 24.9% in JC to 35.9% in BC and 56.7% in MB. Further analysis showed that soil characteristics explained only 19.2% of the AMF community variation among forest types. Hence, vegetation is presumably the main driver of the alteration of the AMF community. The α diversity of AMF was similar between JC and MB, although it was higher in BC. Overall, this research sheds more light on AMF community dynamics during moso bamboo expansion. Our results highlight that the consequences of bamboo expansion in monoculture forests differ from those in mixed forests.