The invention relates to a sewage treatment agent adding device, in particular to a throwing device for a flocculating agent for a sewage treatment tank. The throwing device has a large application range and is capable of rapidly diffusing the flocculating agent in sewage. The throwing device comprises a bottom plate, wheels, a push rod, a mounting block, a curved slide rail, a curved slider, a support rod, a placement sleeve, a push block, a first connecting block, a baffle, moving block, and the like; the lower side of the bottom plate is provided with the wheels, the left side of the frontside of the bottom plate is connected with the push rod, the left side of the bottom plate is connected with the mounting block, and the right side of the mounting block is connected with the curved slide rail. The throwing device can achieve the large application range and is capable of rapidly diffusing the flocculating agent in the sewage. The flocculating agent can be continuously poured intothe sewage by adopting a method using a swinging rod to swing to push the push block to pour the flocculating agent.