The invention relates to the technical field of numerical control machine tools, and discloses an adjustable numerical control machine tool guide rail protection cover. The adjustable numerical control machine tool guide rail protection cover comprises a left cover body and a right cover body. Each cover body comprises two symmetric combined cover bodies on the left side and the right side and an inner core, wherein the combined cover bodies are of a hollow structure, concave-halfpace-shaped guide rail limiting parts are arranged at the bottoms of the combined cover bodies, the inner core is arranged between the two combined cover bodies, transverse sliding grooves are formed in the upper surface of the inner core, sliding rails matched with the sliding grooves are arranged on the inner walls of the combined cover bodies, and the combined cover bodies are in sliding connection with the inner core; and extension parts are further arranged on the upper surface of the inner core and internally provided with cooling liquid ways penetrating the extension parts, the inner ends of the cooling liquid ways are connected with cooling liquid pipelines, the other ends of the cooling liquid pipelines are fixed to the inner sides of the bottoms of the combined cover bodies, and the cooling pipelines are attached to the inner walls of the guide rail limiting parts. The adjustable numerical control machine tool guide rail protection cover has the beneficial effects that the size is small, usage and storage are simple, convenient and fast, the limitation of the application range is small, and the guide rail protection effect is good.