Using GC-O-MS to compare and analyze the change of volatile flavor components in watermelon juice before and after heat treatment. The results showed that 44 and 50 odor compounds were identified by GC-O-MS in fresh and heated watermelon juice, respectively. The key odor compounds in fresh watermelon juice were nonanal (freshness and herbaceous flavor), (E)-2-nonenal (freshness and cucumber flavor), (E, Z)-2,6-nonadienal (freshness and cucumber flavor), (E, Z)-3,6-nonadienol (freshness and cucumber flavor), (E, Z)-2,6-nonadienol (freshness and cucumber flavor) and geranyl acetone (flower fragrance and sweet taste) identified by combination of sniffing, odor dilution factor and odor activity value. However, the key odor compounds in heated watermelon juice were (E)-2-heptenal (fat flavor and bitterness), decanal (fat flavor and soap flavor), 6-methyl-5-hepten-2-one (mushroom and musty flavor), acetophenone (bitter almonds flavor), diethyl disulfide (garlic flavor) and (Z)-6-nonenal (pungent odor). Among them, the characteristic odor of (Z)-6-nonenal was fresh and cucumber flavor initially. Due to the concentration of (Z)-6-nonenal in heated watermelon juice exceeding the tolerance of the sensory assessor's nose, it showed an irritating smell. The heat treatment produced odorous components, and the change in odor compounds led to the cooking taste of the watermelon juice after heating.