This article sets up mathematic model for the traffic circle to determine how best to control traffic flow. In order to make the traffic at the circle intersection smooth in maximum, this model figures out the largest traffic capacity through Wardrop’s formula. When the signal control is needed, a linkage time solving model is set up, which proves the number of seconds of every green light and work out the delay time. At last, the simulation software VISSIM is used for an example to work out the average delay time under the signal control. The result matches with this model.