The invention discloses an open bag filling machine, comprising a feeding pipe and a feeding box, wherein a first feeding port and a first discharge port used for feeding and discharging materials are formed in the feeding pipe respectively, the top of the feeding box is provided with a second feeding port which is positioned below the first discharge port, the feeding box is also internally provided with a vertically communicated distributing pipe which is positioned below the second feeding port, and can be selectively communicated with one of a first passage and a second passage which are positioned below the distributing pipe and are separated from each other, and the bottoms of the first passage and the second passage are provided with a first blanking port and a second blanking port respectively. The open bag filling machine is characterized in that the caliber of the second blanking port is smaller than that of the first blanking port, a fixing mechanism used for fixing an open bag is also arranged below the feeding box, and the fixing mechanism can horizontally move under the first blanking port and under the second blanking port for precisely loading materials into the open bag.