The automatic traffic light controlling system for the crossing consisted of a highway and a farmroad was designed by using VHDL and finite state machine.In the design,the light phase of the green and red light was defined as the state,and the situation of the crossing was defined as the condition,which form a finite state machine.The traffic light controlling system based on the model can make full use of the existed traffic resources,and help to lighten the heavy traffic in the cities.