Background <p>Preservation of residual kidney function (RKF) is critical for patients undergoing peritoneal dialysis (PD). However, the relationship between the rate of RKF decline (RKF slope) and PD technique failure remains incompletely understood, and a clinically actionable threshold is lacking.</p> Method <p>This was a single-centre, retrospective cohort study that included 539 patients who underwent peritoneal dialysis catheter placement at the hospital from 1 May 2006 to 31 December 2022. The primary analysis used pre‑specified Cox models to assess the association between RKF decline rate and technique failure (death or transfer to HD). RCS analysis and Kaplan‑Meier curves were also performed.</p> Results <p>During a median follow‑up of 30 months, 152 patients (28.2%) in the full cohort experienced technique failure. The median RKF decline rate was −0.14 ml/min/1.73 m² /month (IQR −0.20 to −0.08). Faster RKF decline was independently associated with a higher risk of technique failure (per 0.1 ml/min/1.73 m² /month: HR = 1.19, 95% CI 1.05–1.34, <i>p</i> &lt; 0.001). RCS analysis revealed a nonlinear relationship (<i>p</i> = 0.047) with a candidate threshold at −0.20. Patients with decline faster than −0.20 had significantly lower technique failure‑free survival (log‑rank <i>p</i> = 0.027).</p> Conclusion <p>There was a nonlinear relationship between the rate of decline in residual kidney function (RKF slope) and the risk of PD patients experiencing technique failure, with a candidate threshold of −0.20 ml/min/1.73 m² /month. Regularly monitoring the decline in RKF and maintaining it above −0.20 ml/min/1.73 m² /month may help identify patients at higher risk of technique failure, although external validation is needed.</p>