The invention discloses a primer and a method for identifying purity of a cucumber 'Yuexiu No.3' hybrid seed. The method comprises the following steps of: (1) extracting deoxyribonucleic acid (DNA) of cucumber genome; (2) carrying out polymerase chain reaction (PCR) augmentation by using a screened specific primer; (3) carrying out gel electrophoresis on a PCR augmentation product; and (4) judging the purity of the seed according to the relative position on a gel band. By adopting the method disclosed by the invention, the 'Yuexiu No.3' hybrid seed can be distinguished from a female parent seed and a male parent seed; the purity of the hybrid seed can be rapidly detected. The method has the advantages of being rapid, accurate, low in cost, simple to operate and the like, and has high commercial application value; a traditional method for identifying the purity of the hybrid seed can be replaced.