The invention relates to an auxiliary tool for processing modified plastics, in particular to a bag seaming device for packaged plastic granules. A semi-automatic bag seaming device for plastic granules comprises a bag seaming machine, and is characterized in that an upright post is vertically arranged on a plane at one side of a trapezoidal base; a connecting arm is transversely arranged on the upright post; the bag seaming machine is connected with the connecting arm; a row of idler wheels are arranged on a plane at the other side of the trapezoidal base, and outer rings of the idler wheels are provided with conveying belts; the idler wheels are driven by an electric motor which is controlled by a power switch; and the inclined planes at two ends of the trapezoidal base are an upper sliding platform and a lower sliding platform of the packing bags. The upright post has an upper section and a lower section which are sleeved and have adjustable height. The speed-regulating range of the speed of the conveying belts is 0.05 to 0.2 meter per second. The bag seaming device has the advantages that the bag seaming device overcomes the defect of manual bag seaming, has simple structure and low cost, effectively reduces production cost, has a height according with the requirement of human engineering, reduces fatigue and labor intensity of manual hand-held operation, and is favorable for improving seaming operation quality and work efficiency.