Abstract: In order to assess the effect of urban agriculture development on ecological environment, and regulate the structure and function of agricultural ecosystem, dynamic of ecological security of urban agriculture was analyzed. Based on the Pressure-State-Response (PSR) model, 24 indexes were selected from 4 levels to build an assessment system to analyze the dynamic and influencing factors of ecological security of urban agriculture. It could serve as an early warning and system identification on ecological security of urban agriculture. Taking Lanzhou City as an example, this paper researched the ecological security trend at time scale (1989—2009) by PSR model. The results showed that ecological security of urban agriculture in Lanzhou was in level II in 1989, level III in 1999 and level IV in 2009. Index value of ecological security increased by 46.7% in the past 20 years and the status of ecological security was improved gradually. The dynamic degree of ecological response was 31.50% from 1989 to 2009. There were 5 factors which had significant effects on ecological security of urban agriculture in Lanzhou, including primary industry output value, total retail sale of consumer goods, cultivated area per capita, secondary industry output value and net income of farmers. Cultivated land transferring into construction land and unused land was the most important factor for change of ecological security pattern of urban agriculture from 1989 to 2009 in Lanzhou. This empirical study provided an effective way to assess the ecological security level of urban agriculture in ecological fragile region dynamically and construct the early-warning system and ecological security pattern. The dynamic analysis also provided the evidence for monitoring and improving agricultural sustainable development.