To identify the optimal application rate of wall-broken stabilized sludge that achieves significant soil improvement effects, this study investigated the influences of wall-broken stabilized sludge on the physicochemical properties of Shanghai green space soil and the growth performance of ryegrass (Lolium perenne L.). A pot experiment was conducted with ryegrass as the test plant, consisting of 7 treatments with varying sludge application rates: 1) Control (CK1, no sludge application); 2) 5% sludge application (TH1); 3) 10% sludge application (TH2); 4) 20% sludge application (TH3); 5) 30% sludge application (TH4); 6) 50% sludge application (TH5); and 7) 100% sludge application (TH6).?Soil parameters, including pH, water-soluble salt content, organic matter content, hydrolyzable nitrogen content, available phosphorus content, available potassium content, and microbial community composition, were determined. Additionally, key growth indicators of ryegrass were measured to evaluate plant response to sludge application.?The results demonstrated that: 1) Wall-broken stabilized sludge significantly regulated soil physicochemical properties. Soil pH decreased progressively with increasing sludge application rate, whereas the contents of water-soluble salts, organic matter, hydrolyzable nitrogen, available phosphorus, and available potassium increased gradually with higher application rates, though the magnitude of increase varied among parameters. Notably, the accumulation risk of heavy metals (e.g., Hg and Pb) became prominent when the sludge application rate ranged from 50% to 100%. 2) Wall-broken stabilized sludge exerted a "low-concentration promotion and high-concentration inhibition" effect on ryegrass growth. Ryegrass growth indicators reached their maximum values at a 20% sludge application rate, while growth was significantly inhibited when the application rate exceeded 50% (≥50%). 3) Sludge application enhanced the α-diversity of soil microorganisms and induced microbial community succession. Specifically, the relative abundances of halophilic archaeal taxa, bacterial taxa involved in organic matter decomposition and phosphorus cycling, and fungal taxa with wood-decomposing capabilities were significantly increased.?In conclusion, the optimal application rate of wall-broken stabilized sludge for green space soil improvement was determined to be 20%. Application rates of ≥50% are likely to inhibit plant growth due to stress factors . Future studies should validate the long-term effects of this sludge application through field experiments to provide more practical guidance for its application in green space soil remediation.