New energy policies constitute an institutional guarantee for the safe and orderly reduction and substitution of urban fossil energy consumption， and the New Energy Demonstration City Policy （NEP） provides vital support for addressing the dilemmas of urban fossil energy consumption. Based on the panel data of 278 cities in China from 2006 to 2024， this paper integrates scale reduction， intensity efficiency improvement， and structural substitution into a unified analytical framework， and systematically evaluates the multi-dimensional effects and mechanisms of NEP on urban fossil energy consumption. The results show that NEP effectively restrains the total volume and intensity of fossil energy consumption in pilot cities， and accelerates the substitution of fossil energy. Further mechanism analysis reveals that NEP suppresses fossil energy consumption by stimulating green technological innovation， strengthening environmental regulation constraints， and optimizing financial service support. Heterogeneous effects exist due to differences in urban resource endowments， industrial structures， and digital infrastructure construction. The coordinated implementation of the Low-Carbon City Pilot Policy and NEP produces synergistic emission reduction effects on fossil energy consumption. The findings provide theoretical references and policy implications for promoting green energy transition and alleviating fossil energy consumption constraints through pilot policy schemes.