In view of urban natural system as an important carrier of urban socio-economic development, the most important aim of urban natural ecological security (UNES) assessment is to forecast and predict the potential ecological insecurity, thereby contributing to sustainable ecological security management of the life support system. Regarding the shortage of operationality and utility of the current method, a dynamic method of UNES developmental degree assessment was set up in this paper based on the identification of its critical factors and warning of the potential ecological insecurity. Xiamen city was taken as a case to develop a suit of UNES indicator system based on its natural environment characteristics. The results showed that the development degree of Xiamen natural ecological insecurity (NEI) will in blue (mild) warning status and sustainable enlarged trend during 2007-2020. The critical factor will be the external driving force of socio- economic system with continuously increased negative feedback. The degree of ecological insecurity can be ranked as: atmosphere subsystem> water subsystem> land subsystem> biology subsystem. It is necessary to speed up the development degree of regulation measures of these subsystems.