Abstract: The field intensity model uses the concept of physics to represent the tourism influence of node cities with “tourism field intensity” with regarding cities in the urban agglomeration as “field source points”. The interaction between field source points are forming force lines with different strength and the compound field groups are formed by interaction of different force lines with superposition effect. Taking the 26 cities in the Yangtze River Delta urban agglomeration as research scope and the tourism field strength(TFS) as the research object, this paper analyzed the space-time evolution pattern and formation mechanism of regional tourism field strength between 2000 and 2016 by using methods of principal component analysis, accessibility analysis, improved field strength model and geographical detector model. The results showed that: (1)The TFS in the Yangtze river delta urban agglomeration had been continuously enhancing, while its differences between cities became larger since 2000. The traffic accessibility of cities improved significantly and presented obvious “center-periphery” structure. However, the scope of center experienced from original “network” to a closely linked “block” and finally exhibited “branches” along traffic line. As the scope of periphery continuously tended to shrink, the trend of TFS towards equalization and network integration were expected. (2)The spatial pattern of TFS in the Yangtze River Delta urban agglomeration were taking Shanghai as the core and spread along the line of “He-Ning-Hu-Hang-Yong” towards the peripheral “axis”. The growth rate of urban TFS fluctuated in the range of 10.54%~13.89% with gradually enlarge differences and a trend toward polarization. The type of urban TFS constantly evolved from low to high level and generally formed a structure of circle layer centering on high level and higher level cities. (3)The changing spatio-temporal heterogeneity pattern of regional TFS were resulted from the effect of multi-factor cycle accumulation. Specifically speaking, regional transportation advantages, tourism industry development and government policy guidance had significantly strengthening impact on the TFS, regional economic development had a stable influence on the TFS, while the impact of tourism resource endowment on the TFS tended to weaken.Exploring the spatio-temporal differences and formation mechanism of urban TFS is helpful to promote reasonable competition and cooperation in tourism, which contributes to diagnosing the short board and barriers of tourism development and providing scientific basis and practical reference for regional tourism development.