A breathing assistance apparatus comprises a gas supply device, a mask prevent leakage control box (MPLCB), and a mask. The mask is used to cover user's face, and the mask has a gasbag at one side in contact with the user. The gas supply device connects to a gas inlet of the mask through a tunnel, and outputs treatment gas to the user stably. The MPLCB connects to the mask through a windpipe. The MPLCB pressurizes the gasbag automatically when a leakage is detected in the mask, and the gasbag is then inflated to touch the user's face. Therefore, the inside of the mask can remain airtight, so as to prevent the treatment gas in the mask from deflating out of the mask continually.