[Objective] In recent years, widespread branch dieback has occurred in Robinia pseudoacacia protection forest across the Yellow River Delta, potentially associated with rhizosphere soil degradation. This study aimed to elucidate the relationship between dieback symptoms, soil physicochemical characters, and fungal community structure in R. pseudoacacia protection forest. [Methods] Three representative shelterbelt sites (Island, Warhorse Fifth Farm, and Ninth Farm) in Dongying City, Shandong Province, were selected. Rhizosphere soils (0–100 cm) were collected using mechanical stratified sampling. Soil physicochemical characters were determined, and fungal communities were analyzed using high-throughput sequencing. Differences between healthy (HR) and dieback (DR) groups were assessed through PCA, α/β-diversity, and dbRDA analyses to identify key environmental drivers of community variation. [Results] (1) In the DR group, soil pH was significantly higher (P < 0.05), while total carbon (TC), total nitrogen (TN), available potassium (AK), capillary water-holding capacity (CHC), and capillary porosity (CP) were lower than in the HR group. PCA revealed distinct differentiation along two ecological gradients: “bulk density–moisture and organic matter” and “nutrient enrichment–pore structure.” (2) Fungal α-diversity decreased and β-diversity differed significantly (P < 0.05). Community composition shifted from a mutualistic–symbiotic type dominated by Aspergillus and Mortierella in HR to a pathogen–stress-tolerant type dominated by Fusarium, with increased abundance of stress-tolerant genera such as Rhodotorula and Spizellomyces. (3) dbRDA identified soil electrical conductivity (EC) as the key environmental factor influencing fungal community variation (P < 0.05). [Conclusion] There is a strong coupling between the degradation of rhizosphere soil conditions and the imbalance of fungal community structure in Robinia pseudoacacia. Elevated soil salinity may alter soil physicochemical properties and niche competition, thereby driving a community transition from mutualistic to pathogenic dominance.