The invention relates to a molecular marker. Disease-resistant bitter gourd MC18 and susceptible bitter gourd MC 105 are hybridized to construct a bitter gourd genetic segregation group, a simplified genome deep sequencing technology is used to obtain 91856 SLAF labels in total, and 29 difference markers closely related to bitter gourd powdery mildew resistance are screened. The number of the molecular marker is SLAF4542, four primers are used for amplifying a refractory mutation system PCR, during identification of bitter gourd powdery mildew resistance, two specific fragments of 356 bp and 326 bp are amplified in identification of high susceptible bitter gourd and susceptible bitter gourd, and only the specific fragment of 326 bp is amplified in the disease-resistant bitter gourd. The detection rate of the marker in an F2 generation segregation population reaches 98% or more, molecular marker-assisted selection can be carried out on the powdery mildew resistance gene in a bitter gourd breeding material before powdery mildew occurrence in the seedling stage, defects of conventional breeding are effectively overcome, the selection method is simplified, breeding efficiency is improved, and the process of disease-resistant breeding of bitter gourd is accelerated.