Arbuscular mycorrhizal fungi (AMF) play crucial roles in enhancing plant nutrient uptake, improving stress resistance, and maintaining ecosystem functioning. However, the driving mechanisms of AMF community assembly across regional-scale environmental gradients and their relationship with ecosystem multifunctionality (EMF) remain unclear. This study investigated AMF symbionts of two species of Asteraceae (Artemisia frigida, Aster altaicus) distributed across the Xilingol grassland. Through high-throughput amplicon sequencing, we analyzed the AMF community diversity and structural changes in different microbial niches (rhizosphere, endosphere) and host species under environmental gradients (meadow steppe, typical steppe, desert steppe), revealed the process of community assembly and explored the associations between AMF diversity, network complexity, and EMF. We found that microbial niche was the strongest driver of AMF community variation, followed by host species, with geographical distribution contributing the least. Mean annual temperature (MAT), mean annual precipitation (MAP), Normalized Difference Vegetation Index (NDVI), and total phosphorus (TP) were the most important environmental factors constraining the AMF communities of both plants. AMF diversity significantly decreased from the rhizosphere soil to the endosphere. The assembly of AMF species in the two plants exhibited about 70% similarity and 30% dissimilarity. A. frigida demonstrated a stronger host filtering effect. MAT, ammonium nitrogen (NH4+-N), and nitrate nitrogen (NO3--N) potentially drove host plant selection of AMF, while pH, NO3--N, available potassium (AK), catalase (CAT), dehydrogenase (DHA), and sucrase (SC) likely influenced host filtering of AMF. Furthermore, stochastic processes dominated AMF assembly in both the rhizosphere and endosphere. However, from meadow steppe to desert steppe, the contribution of deterministic processes increased with increasing environmental stress. Heterogeneous selection and dispersal limitation dominated in the rhizosphere, while homogeneous selection and drift dominated in the endosphere. Additionally, AMF network complexity decreased from rhizosphere to the endosphere, and the AMF network complexity of A. altaicus was higher than that of A. frigida. With increasing environmental stress from meadow steppe to desert steppe, AMF community diversity decreased, networks gradually simplified, and ecosystem multifunctionality weakened. Besides, we confirmed the correlation of AMF diversity with ecosystem multifunctionality, yet insufficient evidence proves causality between the two. Moreover, climatic factors (MAP, MAT) had direct effects on EMF. The findings provide further insights into the driving patterns and mechanisms underlying the assembly of AMF communities in temperate grasslands.