This paper develops a novel hybrid Trigonometric Envelopment Analysis for Ideal Solutions (TEA-IS) model to assess the ecological efficiency of 248 Chinese cities for a 14-year period and employs a difference-in-difference approach to reveal the mechanism of the impact of low-carbon pilot city policies on ecological efficiency from the perspective of green technology innovation. This TEA-IS approach not only combines the advantageous features that exist in each individual model, but also allows an analysis of ecological efficiency in terms of the synergistic effects that may exist among correlated positive and negative criteria. Machine learning methods are also employed to predict high-low performance and synergistic Chinese cities in terms of the socio-economic and demographic context. Results indicate that ecological efficiency is generally low in Chinese cities. The low-carbon pilot city policy does help to improve ecological efficiency, which results from the mechanism of green technology innovation.