The invention provides a method for amplifying four genes of Botrytis cinerea and a multiplex PCR (polymerase chain reaction) primer set thereof, relating to the field of molecular biology. On the basis of drug-resistant mutant sites of known Botrytis cinerea p-benzimidazole, boscalid, iprodione, fenhexamid and many other bactericides, a PCR primer set for simultaneously amplifying four gene segments beta-tub, BsOS1, erg27 and SdhB is established. The extracted Botrytis cinerea genome can be directly used as a template and subjected to amplification by using the designed primer set to obtain the enriched four gene segments. The multiplex PCR process is used for the first time to simultaneously amplify the four drug-resistant mutant genes of Botrytis cinerea, thereby greatly reducing the workload for drug-resistant site detection in the later period. By optimizing the PCR reaction system, the four gene segments are amplified in one PCR process.