The invention discloses a molecular marker related to tomato powdery mildew resistance and application of the molecular marker. The method comprises the following steps: detecting whether the 100th-101st nucleotides of SEQ ID No.4 in a tomato genome are deleted or not, identifying or assisting in identifying the powdery mildew resistance of the tomatoes according to the genotype, and breeding the tomatoes, the genotype of which is GG or GT, to be detected is a tomato powdery mildew resistant tomato or a candidate tomato powdery mildew resistant tomato; the to-be-detected tomato with the genotype of TT is or is candidate to be a powdery mildew-infected tomato; the powdery mildew resistance of the to-be-detected tomato with the genotype of GG or GT is higher than that of the to-be-detected tomato with the genotype of TT. The substance for detecting whether the 100th-101st nucleotides of SEQ ID No.4 in the tomato genome are deleted or not can be combined with other substances (such as substances for detecting single nucleotide polymorphism or genotype of other molecular markers related to tomato powdery mildew resistance) to prepare products for identifying tomato powdery mildew resistance varieties.