A convertible circular knitting machine having mechanical main body, cylinder mechanisms, yarn feeders, and a fabric winding system. The cylinders are mounted on the top of the base of the main body. The upper and lower cylinder mechanisms are driven by synchronous shafts. The yarn is transmitted from yarn feeders to cylinders and then knitted to clothes and wound by the fabric winding system. The apparatus is characterized in that upper and lower cylinder mechanisms are all interchangeable so that the upper cylinder can be replaced with radial dust cleaning system. In addition, an upper support rack and a set of rings for feeding thread can be mounted on the base. Therefore, the present invention has multi-functions, wherein the upper and lower cylinders can be adjusted or replaced, or added with optional elements such as an upper support rack, rings for feeding thread and radial dust cleaning system in order to convert the model of producing fabric and offer a wide range of applications.