The invention provides a microwave-absorbing patch and a preparation method thereof. The preparation method comprises the following steps: combining butyl rubber and modified boron carbide at a certain ratio; adding an appropriate amount of a vulcanizing agent, a softening agent and an accelerating agent; and separately mixing and vulcanizing to prepare the microwave-absorbing patch. The preparation method provided by the invention prepares the microwave-absorbing patch having excellent mechanical properties while having excellent microwave absorbing properties by mixing boron carbide with butyl rubber, and solves a problem of poor properties of existing microwave-absorbing sheets; the microwave-absorbing patch has light mass and has an absorption peak at a lower frequency position, thereby solving problems that a magnetic absorbent cannot solve; therefore, the microwave-absorbing patch provided by the invention fully meets requirements of 'thin, light, wide and strong' required by a stealth technology, can be widely applied to aerospace stealth, electronic integrated circuits, electronic appliances and other high-temperature and anti-interference equipment, and can also be appliedto anti-information leakage, medical detection, microwave absorption detection and other microwave absorption darkrooms.