Provided in the disclosure is a photomask for detecting flare degree of lens of exposure machine. The photomask includes a central exposure area and a peripheral area, exposure light of the exposure machine passing through the lens and then penetrating the central exposure area to expose photoresist on a wafer, wherein the entire central exposure area is provided with a shading layer to prevent the exposure light from penetrating; and the peripheral area is provided with a plurality of light-transmitting stripes, and stray light formed after the exposure light passes through the lens penetrates the plurality of light-transmitting stripes to expose the photoresist. Further provided in the disclosure is a method for detecting flare degree of lens of exposure machine by using the photomask. According to the disclosure, a lens flare problem of an exposure machine can be found and solved in time.