A Venturi tube assembly includes: a Venturi tube having a body section with an inlet end, an outlet end and an internal flow passage between the inlet end and the outlet end, a pneumatic sucking passage outward extending from a low static pressure section of the internal flow passage to an outer circumference of the body section; and a blocking member having an exhaustion space for communicating the outlet end with the atmosphere, a blocking body movable between an unblocking position and a blocking position and a micro-section movable between a full open position and a micro-position. When positioned in the full open position, the exhaustion space fully communicates the outlet end with the atmosphere, whereby full amount of airflow is exhausted from the outlet end to the atmosphere. When positioned in the micro-position, only a micro-amount of airflow is exhausted from the outlet end to the atmosphere.