A disposable thermometer probe sheath includes a bottom piece and a plastic cover film sealed onto the bottom piece. The plastic cover film comprising an upper plastic sheet film and a lower plastic sheet film, each of which has a front edge provided with an opening, a rear edge provided with a tip, and a sealed peripheral edge except for the opening, so as to receive a thermometer probe. The plastic cover film is made of a flexible and deformable material, while the plastic cover film is provided with two notches respectively formed on two side edges adjacent to the tip. The upper plastic sheet film is provided with a first flap attached to the opening thereof, while the lower plastic sheet film is provided with a second flap attached to the opening. When the thermometer probe is inserted into the plastic cover film, the notches are tightly engaged with the thermometer probe. When separating the thermometer probe from the plastic cover film, one of the upper plastic sheet film and the lower plastic sheet film of the plastic cover film is torn from the opening thereof along a longitudinal direction of the plastic cover film under the assistance of the engagement between the two notches and the thermometer probe, and then the one of the upper plastic sheet film and the lower plastic sheet film will be overturned from the opening toward the tip of the plastic cover film, so that the thermometer probe is separated from the plastic cover film.