It has been observed that the physical extent of urban areas typically overflow the administrative limits of single cities in Mainland China. Also, the extent of territories, or hinterlands, have not been considered in regional statistical analyses and management. While many existing case studies have investigated the functional urban areas of single cities, the problem of how to delineate urban areas in geographic space relating to large bodies of cities or at the scale of an entire country has not been investigated. In this work, it is suggested that big data can provide great opportunities for the delineation of functional urban areas at larger scales. In this study, over 30 million anonymized 2016 car-hailing records were collected from Didi Chuxing, the largest car-hailing online platform in the world (to the best of our knowledge). A core-periphery approach is then proposed that uses nationwide and fine-grained trips to understand functional urban areas in Mainland China. This study examines 4,539 out of all 39,007 townships in an attempt to provide a new method for the definition of urban functional areas in Chinese Mainland. In addition, four types of cities are identified using a comparison of functional urban areas with their administrative limits, and a further evaluation is conducted using 23 Chinese urban agglomerations. This study is needed for the quantification and understanding of Chinese city development and to provide a basis for city planning, management, and statistical analyses.