In the context of the era of carbon neutralisation and carbon peaking,improving urban carbon emission efficiency significantly contributes to the realization and development of low-carbon city construction.Urban compactness is closely related to carbon emission efficiency due to its ‘form,structure and function’,and it is important to demonstrate whether urban compactness can improve carbon emission efficiency and explore its spatial spillover characteristics. Based on the panel data of 279 prefecture-level cities in China,the Herfindahl index and the super-efficiency SBM model are used to measure the urban compactness and carbon emission efficiency of the prefecture-level cities to explore the spatial evolution trend and correlation characteristics of the two,and apply the dynamic spatial Durbin model to analyse the spatial spillover effect of urban compactness on carbon emission efficiency. The results show that: ① The urban compactness in each city tends to change in the polycentric structure,with a spatial pattern that is typically "high in the south and low in the north". The areas with extremely high values are predominantly found in the core cities of major urban agglomerations in China. Carbon emission efficiency level generally exhibits a steady upward trend,with the northern segment of the high-value urban agglomeration area progressively moving southward. ② According to the estimation results of the dynamic SDM model under the spatio-temporal fixed effect,there are significant positive spatial spillover,temporal inertia and spatio-temporal monsoon effects of urban compactness in geographically close areas. This is demonstrated by the fact that the regulation of urban compactness leads to upward and downward fluctuations in the efficiency of carbon emissions across regions.③ The decomposition of impact effects indicates that while a polycentric structure significantly enhances a city's carbon emission efficiency,it can negatively affect neighboring cities through spatial spillover,diminishing their carbon emission efficiency. The phenomenon of ‘excessive polycentricity’ in 83 cities in the East under spatial heterogeneity makes the impact of urban compactness on local carbon emission efficiency ‘from facilitating to suppressing’.④ In terms of policy formulation,it is recommended that a multi-centre spatial structure be established in a gradual and orderly manner to further improve energy efficiency; that market integration be further promoted,market barriers be eliminated and the free flow of factors be facilitated; and that transport infrastructure be strengthened and intercity transport costs be reduced to promote a more positive role of the multi-centre spatial structure in terms of energy efficiency.