Cities are the gathering place and essential carrier of human production and life, it is also the main subject of energy consumption and carbon emission. Thus improving carbon productivity is the most important starting point in the policy toolbox of building low-carbon cities and developing green economy. This paper built a theoretical model of the relationship between urbanization and urban carbon productivity based on the Cobb-Douglas cost function, then tested the dynamic relationship between population urbanization, land urbanization and urban carbon productivity at the level of China"s Prefecture-level Cities with threshold regression model. The empirical results show that population urbanization and land urbanization have different staged effects on urban carbon productivity. Specifically, the relationship between population urbanization and urban carbon productivity presents a "strong promotion-weak promotion" relationship with the inflection point of 5.36% of population urbanization rate, while the land urbanization and urban carbon productivity showed a "promotion-decoupling" relationship with the inflection point of 1.09% of the land urbanization rate. The regional empirical results prove the robustness of the research conclusions from the regional level. The results reveal that most prefecture-level cities in China have passed the two thresholds, which means that low-carbon urbanization and reasonable urban planning will be the only way to develop green economy in China’s cities.