The present invention discloses a ventilator with dual flow passages. There is a mode switcher on the partition board. The ventilator has a variety of operating modes, such as, the natural or mechanical driving mode according to outdoor wind speed, and single-effect or multi-effect filtration according to the outdoor air pollution conditions. The single-effect filtration uses only the primary-efficiency filter, while the three-effect filtration uses the primary-, medium-, and high-efficiency filters. The mode switcher on the partition board can open or close the by-pass baffle to bypass or go through the medium- and high-efficiency filters for switching between the single-effect and multi-effect filtration. In case some indoor pollution sources need to be removed quickly, the recirculated indoor air filtration can be run. In such situation, the primary-efficiency filter in the mechanically-driven flow passage is bypassed, while the fan drives indoor air passing the medium- and high-efficiency filters.