A handle having an airtightness function used for a casing of a negative pressure type dust collector is provided. Two pivot seats are fixed to a door of the casing and vertically aligned with each other. A rotating shaft is pivotally connected to the pivot seats. Two ends of the rotating shaft are provided with eccentric blocks, respectively. Two engaging blocks are located close to the outer periphery of the door and vertically aligned with each other. The eccentric blocks are engaged with the engaging blocks to exert a force to the door so that the door is closely attached to the casing, thereby enhancing the airtight effect of the casing and preventing the door from being deformed due to backwash and preventing the dust from flowing out through the gap between the door and the casing to cause secondary environmental pollution.