This paper analyzes the time constant effect of room temperature sensor on the VAV system. HVACSIM+(A simulation program of HVAC system) is used as the platform of the simulation. In the simulation, two kinds of VAV terminals (pressure dependent and pressure independent) are used. Different simulation results of two kinds of VAV terminals with different time constant are presented. The results show that we can use pressure dependent VAV terminals with different time constant are presented. The results show that we can use pressure dependent VAV terminal to get satisfied control results approximately if the time constant of room temperature sensor is less than 30 second. Because the VAV terminal is very expensive in these days and the pressure dependent terminal is cheaper than the pressure independent terminal by one third or so , we can use the cheaper terminal if the time constant of room temperature sensor can be small .The time constant of one real room temperature sensor of one VAV terminal is measured .The result is that the time constant is too great because of the thermal inertia effect of the microenvironment surrounded by the wall and the enclosure of the temperature controller . A new method of the temperature sensor position in the controller is given out here to decline the time constant of the controller in sensing the room temperature change. By decline the time constant of the room temperature sensor, we can use pressure dependent terminal to cut down the cost the VAV system.