China is experiencing rapid urbanization. It is of great significance to clarify the characteristics of urban spatial expansion and its evolution law for regional coordinated development. Urbanization involves many aspects such as economy and population. However, current researches still face challenges in comprehensively reflecting the spatial pattern, evolution process, and intrinsic mechanism of regional urbanization, and they cannot effectively reveal and quantify the spatial correlation and evolution process of regional urbanization. Based on the data of economy, population and urban space in the Yangtze River Delta region from 1995 to 2015, this paper used spatial polarization and diffusion theory to quantitatively analyze the spatial evolution process and characteristics of urbanization. The results showed that: (1) The characteristics of urbanization level in the study area were obvious. Shanghai was at the first level, Nanjing, Hangzhou and Suzhou were at the second level, and the rest of the cities were at the third, fourth and fifth levels. The spatial correlation intensity of urbanization shows that the high-value level was concentrated in the eastern part of the region and 10 cities, such as Shanghai, Suzhou and Hangzhou, constituted the most intensive urbanization development spatial network. (2) During the study period, the spatial polarization and spatial diffusion characteristics existed simultaneously in the Yangtze River Delta region. During the study period, a diffused region always rapidly presented a patttern of polarization in space, then the polarization weakened and turned into a new diffusion phase. It is most obvious in Nanjing and Hangzhou among all the 41 cities. Our findings completely demonstrate that the spatial correlation among cities is changing over time. This study also explored the driving mechanism of regional urbanization, indicating that industrial upgrading and transfer within the study area is the main endogenous driving force for spatial polarization or diffusion. Our study has important theoretical and practical value for future regional development planning.