The invention discloses ultrafine active calcium silicate powder and a production process thereof. The ultrafine active calcium silicate powder is prepared from the following raw materials including basic silicon-containing recovery liquid with the concentration being 0.4 to 2.4 percent, hydrochloric acid solution with the concentration being 2 to 10 percent and calcium chloride solution with theconcentration being 10 to 50 percent. The product is prepared from byproducts in the production process; the resources are comprehensively utilized; the production cost is reduced; the environment pollution is reduced; the product has the characteristics of high purity, fine granularity and the like; the powder granularity is 325 to 2500 meshes; siloxy functional groups identical to white carbon black exist on the surface; high reinforcing performance is realized; the production cost is only 1/4 of that of white carbon black. The mass application to rubber and plastic industry as reinforcing agents and wide application to industrial departments such as medicine, pesticide and ink as additives are realized.