Sandy ecosystems have low nutrient availability and strong environmental stress, so vegetation restoration in these areas depends greatly on plant-microbe interactions. Mixed plantations are widely used to improve ecosystem stability in desert regions, but their effects on fungal community assembly along the root-soil continuum are still not clear. Here, we investigated fungal communities associated with Populus simonii monocultures and three mixed plantations (P. simonii * Salix psammophila, P. simonii * S. psammophila, P. simonii * Caragana korshinskii, P. simonii * Pinus sylvestris var. mongolica) in the Kubuqi Desert of northern China. We used high-throughput sequencing of the fungal ITS region together with functional guild prediction, ecological network analysis, and null model analysis to study community diversity, assembly processes, and environmental drivers across root (R), rhizosphere (RS), and non-rhizosphere soil (NRS) compartments. Mixed plantations significantly altered soil physicochemical conditions and reshaped fungal community structure along the root-soil continuum. RS harbored the highest fungal richness, whereas root-associated communities exhibited strong compositional differentiation, indicating pronounced host filtering effects. Ectomycorrhizal fungi dominated the fungal functional structure, suggesting that symbiotic strategies play a central role in plant adaptation to nutrient-poor sandy environments. Mixed stands also increased fungal network complexity, particularly in the PC stand, implying stronger microbial interactions and potentially greater ecosystem stability. Null model analysis revealed that stochastic processes, especially dispersal limitation, predominantly governed fungal community assembly, while deterministic selection became more important in the rhizosphere. Soil water content and available potassium emerged as the key environmental drivers shaping fungal community composition. Overall, our findings demonstrate that mixed plantations regulate fungal community assembly through both plant-mediated microbial filtering and modifications of soil environmental conditions. These results highlight the importance of species mixtures in structuring belowground microbial networks and provide new insights into the microbial mechanisms underlying vegetation restoration in sandy ecosystems.