The urban classification is a major practical and theoretical issue，especially in the digital era，when the functions and forms of cities are undergoing a turning point，which is causing a reshaping of the urban form and the functional structure within the global urban system. Based on the theory of substitution elasticity，this paper argues that the status of a city in the global urban system is determined by the substitution elasticity of the city，and the substitution elasticity of the city is determined by the degree of agglomeration and the degree of connectedness of the city together. Subsequently，this paper constructs a system of indicators to measure the degree of agglomeration and connectedness of cities，and carries out the classification and type of global cities. According to the research of this paper，the global city system is a multi-layer nested structure，and the 1006 sample cities can be divided into 3 layers，2 types，5 classes and 10 levels. In terms of types，there are six categories of cities：high agglomeration and high connectedness，high agglomeration and medium connectedness，high agglomeration and low connectedness，medium agglomeration and medium connectedness，and low agglomeration and low connectedness. It is found that agglomeration and connectedness are positively correlated and that agglomeration is a prerequisite for connectedness. Europe has a significant number of cities in the high agglomeration-high linkage and medium agglomeration-medium linkage categories，which are important for leading and supporting its future development. Africa has a large number of low agglomeration and low linkage cities and is lagging behind in its overall stage of development. Asian cities are highly differentiated，with outstanding achievements，significant problems and great potential. Osaka is the only city with high agglomeration and low connectivity，and its experience and lessons learnt should be taken into account by other cities around the world.