The short-time aerobic digestion (STAD) process has been found to be a unique and significant technique for the stabilization of waste activated sludge (WAS), but the influences of the system pH on the STAD process was unclear. This study systematically disclosed the influences of the system pH on the STAD process of WAS. Under neutral or weak alkaline conditions, although the biodegradation rates of VSS (~0.0085 h⁻¹) were low, high biodegradation rates of TCOD (k_TCOD) (~0.0096 h⁻¹) were achieved. Less releases of the biopolymers from the WAS led to low concentrations of STOC, UV₂₅₄, the low MW organic matters, NH₄⁺ ‐ N and PO₄³⁻ ‐ P in the supernatant. However, the appropriate pH for the microorganisms improved SOUR, indicating that the released substances were further reused or biodegraded by the microorganisms. Under acidic or alkaline conditions, the biodegradation rates of VSS (0.009–0.019 h⁻¹) and TCOD (k_TCOD) (0.005–0.009 h⁻¹) were opposite with those under neutral or weak alkaline conditions. The releases of the biopolymers were increased, leading to high concentrations of STOC, UV₂₅₄, the low MW organic matters, PO₄³⁻ ‐ P and NH₄⁺ ‐ N in the supernatant. However, the extreme pH inhibited the microbial activity. The SOURs were only 0.0097 h⁻¹ and 0.0053 h⁻¹ for system pH of 8.0 and 4.0, respectively. Accordingly, neutral and weak alkaline conditions should be more suitable for the STAD process of WAS. This work lays the foundation for optimizing system pH for the reduction of WAS in STAD system.