With rapid urbanization in China, there is an urgent need to better understand spatial patterns of cities’ carbon transitions in urban metabolism processes and to explore ways to achieve low carbon cities through sustainable urban planning. In this study, we took Hangzhou City as an example, and firstly analyzed changes in carbon emission and sequestration based on land use types, using data with 5 years interval from 1995 to 2015. We then quantified both harmful and beneficial carbon transitions by land use changes spatially. Harmful carbon transitions are processes that cause carbon sequestration decreases or carbon emission increases and the beneficial ones are the opposite processes. Finally, we employed a panel data regression analysis to investigate how urban forms and road structures influence urban carbon emission at the district level. We found that carbon emissions experienced a nearly five times increase, while carbon sequestration decreased by 22.29%. And the land use change from cultivated land to industrial land accounted for 61.05% of the harmful carbon transitions. In addition, the spatial analysis of urban carbon transitions indicated harmful carbon transitions always sourced from the eastern parts of Hangzhou. Most beneficial carbon transitions were located in the urban central area and western mountainous areas. The panel data analysis demonstrated that urban expansion modes with high connectedness and a better coupling relationship between urban form and road structure could help to emission reduction, and a mononuclear urban expansion mode at the district-level exerted a positive influence in relation to carbon emissions increase. This study provided an approach to assess urban carbon metabolism and help to better understanding of low-carbon urban form.