The invention relates to a special protector and a method for a speed reducer of a downhole electric submersible progressing cavity pump.The method includes that new lubricating oil is delivered into downhole environments via a special oil injection cable by a high-pressure oil injection pump on the ground, a check safety valve is arranged at the lower end of the special protector, starting pressures of the check safety valve can be adjusted according to different well depths, the check safety valve is started under the effects of oil pressures, the new lubricating oil is injected into the special protector from the lower portion, and old lubricating oil is drained into clearance of oil casings via a breathing hole in the upper portion of the special protector, so that an oil change procedure can be completed.The special protector and the method have the advantages that the special protector is high in original protector compatibility, and most accessories are universal; the special protector is simple in structure, and the starting pressures of the check safety valve can be adjusted according to the different well depths; the special protector can be easily and reliably connected with the oil injection cable; the lubricating oil can be changed periodically, accordingly, the downhole speed reducer can be assuredly excellently lubricated, and the service life of the speed reducer can be effectively prolonged; the thrust bearable capacity can be enhanced; the pump checking cycle can be prolonged, and the operating expense can be reduced.