It is of great significance, both theorefically and practically, to study the change of the port-city relationships. Under the background of globalization, the relationships between ports and cities have become more complicated and subtle than at any time, and can be summarized as follows: Firstly, the relationships based on handling cargoes have decreased, while financing trade relationships are increasing. Secondly, the change tendencies of different ports-cities may vary from one to another. In this paper, the author first analyzes the associative structure between port and city, and then use gray correlative analysis to measure the effect of Nanjing port-city interactive development under the background of globalization. A practical formula of the mean associative degree (MAD) is designed for determining the direction and degree of dynamic change of the port-city associative degree. By calculating the concerned statistics from 1990 to 2001, it can be clearly seen that MAD in 1990 is 0.68679, which is relatively high, then there appeared a steady tendency to drop. In 1994, MAD reached its minimum which is 0.63763, following a steady tendency to rise. In 1998, MAD reached its maximum which is 0.8853. But then an evident drop took place. According to these results, the authors hold that there are two obvious features of the effect: (1) On the whole, the effect of Nanjing port-city interactive development is relatively notable. (2) There are obvious differences in the change tendencies of the effect in the 3rd phase.A systematic analysis has been carried out on the formation causes of the effect change of Nanjing city-port interactive development. This paper holds that the following factors have profound implications on it: the change of industrial structure of Nanjing, the changing function of Nanjing port, the intense competition between the ports all around, the administrative system of port and the development of international trade.