The invention provides a protective device with variable diameter. The protective device with variable diameter comprises a fixed disc 1 and a variable-diameter guard shield 7, wherein the variable-diameter guard shield 7 is a shield body consisting of a plurality of variable-diameter units 701; a sliding mechanism is arranged on the fixed disc 1 corresponding to the variable-diameter units 701; the sliding mechanism is fixedly connected with the variable-diameter units 701; the sliding mechanism drives the variable-diameter units 701 to stretch to change the radius/diameter of the variable-diameter guard shield 7. The diameter of the protective device with variable diameter provided by the invention can be adjusted automatically, and the protective device is accurately attached to an outer table of a mould. A manipulator in the shield automatically cleans the mould. The protective device with variable diameter has the advantages of convenience in mounting and use, low cost and wide application range.