1. In a computer system, a method for performing a network planning of a mobile communication network comprising the steps of: importing a first data set into a memory of the computer system, wherein the first data set represents a network topology of the mobile communication network; collecting a second data set into the memory, wherein the second data set represents a plurality of statistic records, and the statistic records reflect a plurality of user mobility behaviors and a plurality of traffic behaviors of network elements in the mobile communication network; creating a first network plan through processing the first and the second data set by a processor of the computer system; displaying the first network plan through a graphic user interface (GUI) on a display device of the computer system; defining a planning scope representing a network area desired to be executed the network planning and mobility management optimization by the processor; setting a time period for a sample record from the second data set by the processor; processing the sample record by the processor; constructing a mobility model according to the processed sample record by the processor, wherein the mobility model represents the user mobility behaviors within the planning scope during the time period; and obtaining a plurality of traffic loads as a preliminary result of the first network plan by the processor.