The invention discloses a special foaming agent for a hard PVC (polyvinyl chloride) low-foaming material and a preparation method thereof. The special foaming agent comprises the following materials in parts by weight: 20-80 parts of azodicarbonamide (ADC), 20-80 parts of auxiliary foaming agent, 1-20 parts of activating agent, 0.2-10 parts of dispersing agent and 0.2-5 parts of processing agent. The preparation method comprises the steps: adding the dispersing agent and the processing agent in a mixer, mixing for 3-7 minutes, and sequentially uniformly mixing the ADC, the auxiliary foaming agent and the activating agent in the mixer to prepare the special foaming agent. The special foaming agent is suitable for hard PVC low-foam materials such as PVC crushing and low-foaming sectional bars, tubular products and sheets, a wood-plastics low-foaming material and the like. The special foaming agent has the positive effects that 1, the decomposition temperature is 160-190 DEG C, and is within a processing temperature range of the hard PVC low-foaming material; 2, the foaming process is short in induction period, the decomposition is performed without mutation, and the foaming process is easily controlled; 3, the hard PVC low-foaming material is used on a hard PVC low-foaming product, and has a smooth surface and has no discoloring and ageing phenomena; and 4, the special foaming agent is low in cost and good in comprehensive performance, and accords with the use requirement.