Abstract: As a result of development of globalization at the high end, global cities are usually built on the network of global transportation and telecommunication. The network started from a regional network between cities to a city network within a nation, and ended in the global city network. In such a development, international trade plays an important role for connecting cities at different levels. A city network at higher levels is the accumulation of the networks below it. For example, the global city network is a pyramid of the accumulation of the global city network in a narrow sense, national city networks and regional city networks. Accordingly, regional cities, national cities and global cities are cultivated at respective levels. For a city network of high ranks, the layout of the network consists of cities at its level and below. For this reason, the hierarchical global city network consists of regional city networks, national city networks and the global city network in a narrow sense. Meanwhile, global cities stand at the top of the pyramid. Based on the discussion above, we finally come to the law behind the changes of global cities, which we should follow in the construction of global cities.