A computer aided design system comprises an interface creating module, a first calculating module, a dividing module and a second calculating module. The interface module creates a parameter setting interface to display the proposed design on the screen of the device formed with nets and cline segments and select at least one net in response to the user's operation. The first calculating module calculates the length of the cline segments of the potential net in order based on the coordinates of the cline segments and generates a dividing signal. The dividing module divides the cline segments into a first team and a second team based on the compared result with a predetermined width according to the dividing signal. The second calculating module adds the calculated cline segments length in the first team and in the second team to obtain a first length and a second length.