Does the “Zero-waste City" Pilot Improve Urban Green Innovation Efficiency? Evidence from a Multi-period Difference-in-Differences Analysis of Chinese Cities

Solid waste is no longer just an end-of-pipe problem, and whether the broader policies built around it actually drive green innovation is still an open question. We examine how China’s Zero-waste City Pilot (ZWCP) affected urban green innovation efficiency (GIE) and through what channel. Using a balanced panel of 210 Chinese prefecture-level cities for 2012–2023, we applied the Callaway and Sant’Anna (2021) doubly robust difference-in-differences estimator. We measured GIE with a two-stage slacks-based DEA model that splits the score into knowledge production efficiency (E₁) and economic conversion efficiency (E₂). The pilot’s effect was real but short. In 2020, the first batch’s launch year, the calendar-time ATT reached 0.0330 (p = 0.043; about 3.35%), with gains in both E₁ (+4.64%) and E₂ (+2.26%). The effect faded after 2021, and the overall ATT was close to zero. The industrial solid waste comprehensive utilization rate rose by 5.93 percentage points (p = 0.001) and kept climbing, but its path did not match GIE’s. That gap suggests it serves as a direct policy target rather than a causal channel. A Baron–Kenny mediation analysis with 1,000 city-clustered bootstrap replications points instead to industrial restructuring. The pilot reduced the tertiary-to-secondary output ratio (a = −0.194, p < 0.01), which accounted for about 17% of the GIE effect (indirect 95% CI [0.0008, 0.0257]); the direct effect was not significant. The first wave of regulation seems to trigger sector reallocation, not lasting R&D incentives. Future policy needs to turn these early gains into lasting green innovation capacity.