When the roots blower has went through improvement by VFD, the system can realize soft starting and soft stopping, so as to reduce the shock of the system to the power grid in start-up, thus reducing the system start-up times and ensuring a stable operation. What's more, because of the deceleration of the revolving speed in roots blower operation, it can reduce the mechanical wear and prolong the operating life of both the motor and the roots blower. If the system employs pressure closed-loop control improvement, the fully automatic control and unattended operation can be realized.