With the advancement of social technology and the deepening of market economy, consumption flow has shown its advantages in reflecting the degree of regional economic integration. Based on the China UnionPay's intercity consumption flow data in 2018, this paper applies Head-Mayer border effect model to measure and evaluate the degree of regional economic integration of the Yangtze River Delta City Group. The empirical results show that: 1)The unified market in the Yangtze River Delta region has not yet been fully formed. Both urban and inter-provincial boundaries have significant barrier effect on off-site consumption. 2)The inter-provincial boundary effect shows heterogeneity, in which the boundary between Shanghai and the other three provinces has a relatively weak impact on the consumption flow, while the market segmentation between Zhejiang, Jiangsu and Anhui provinces is more obvious. 3) The degree of border effect varies from city to city in the Yangtze River Delta. The intensity of the urban boundary effect in Shanghai is 1.05. The border effect of the cities in Jiangsu Province is between 3.06 and 5.51, the border effect of the cities in Zhejiang Province is between 3.13 and 6.24, and the border effect of the cities in Anhui Province, which is the most intense, ranges from 3.85 to 6.58. Based on the current situation of regional economic integration in the Yangtze River Delta, the following suggestions are put forward: first of all, the role of regional economic coordination organizations in promoting regional economic development draws our attention. Secondly, we should continue to deepen the reform of removing the artificial obstacles that affect regional economic development and further reduce the obstacles and costs of economic cooperation. Lastly, we need actively learn from the experience of international regional cooperation to promote the development of the Yangtze River Delta.