Long-term continuous cropping often degrades soil and reduces productivity, yet how soil properties and microbial communities respond in continuous tree peony monoculture remains poorly understood. Herein, soil physicochemical properties were investigated across a monoculture chronosequence of 1, 3, 5, 8, and 10 years of continuous tree peony cropping, together with rhizobacterial and fungal community diversity, composition, and predicted functions. The contents of soil organic matter, total nitrogen and available nitrogen increased along the chronosequence, whereas the contents of available phosphorus declined sharply after 3 years and remained low thereafter. Bacterial richness declined monotonically, whereas fungal richness reached its lowest level at 8 years. Beta-diversity revealed a clear community succession, with Acidobacteriota suppressed, and fungal communities shifted directionally from Fusarium to Phoma and ultimately to Monographella in the 10-year fields. Predicted functional profiling suggested a potential metabolic shift from energy-intensive pathways to stress-adaptive processes, including enhanced membrane transport. Redundancy analysis and structural equation modelling further indicated that pH and available phosphorus were strongly associated with microbial assemblages, with monoculture duration indirectly linked to communities through alterations in these soil properties. Overall, phosphorus availability, rather than bulk organic matter accumulation, emerged as the primary factor shaping microbial community dynamics and pathogen succession in this alkaline agroecosystem. These findings point to stage-specific pathogen control combined with phosphorus optimization as a potential strategy to mitigate replant disease in tree peony monoculture systems.