The present invention relates to a heat-isolating cup sleeve, which comprises a main body and at least one fixer disposed on the main body and able to be folded inwards to insert between a cup and the cup sleeve. The present invention employs a die-cutting part and folds inwards the die-cutting part to form the fixer similar to a wedge, so as to increase inner tensile forces of the cup sleeve sleeving a cup so that the cup sleeve is in a tensioning state and closely attaches with the cup. Therefore, the cup sleeve is difficult to slide from the cup. When the beverage is cooled, the cup sleeve is conveniently taken down from the cup, and is simple and environmental.