[Objective] Measure and analyze the optimum population of Xi'an City from 2001 to 2011, in order to discuss the characteristics and the cause of the change in the population carrying capacity, and to put forward the optimization countermeasures.[Methods] Four models, which are food carrying capacity, ecological footprint, improved economy carrying capacity and improved comprehensive carrying capacity were used.[Results] The population size and structure of Xi'an City presented periodic characteristics in different phases from 2001 to 2011. Since 2004, population carrying deficit has always existed, and with this situation, the trend of it became smooth after a rapid growth stage. The economy carrying capacity of Xi'an City was stronger, however, the food and ecological population carrying capacity was insufficient.[Conclusion] In the process of improving the area comprehensive carrying capacity of Xi'an City, the land carrying capacity is the base, the economy carrying capacity is the key point, and the ecology carrying capacity is the limitation.