This paper investigates the necessity and property of building cross — domain urban subway transit in China from the field theory. Then put Beijing, Yanjiao and Sanhe for example to construct the time coordinate model, which is suitable to China's national condition, by the properties of the field decreasing with time distance to illustrate the basic function of cross domain city rail commute in metropolitan area construction. Finally, we extended the conclusion on other Chinese metropolitan construction and how to alleviate the problem of ghost town then put ‘give priority to urban rail transit’ as a policy suggestion.