Fuente: WIPO "tobacco agriculture" The invention provides a co-dominant SSR (Simple Sequence Repeat) marker closely linked with an NNK release amount gene qNNK in mainstream smoke of tobacco and application of the co-dominant SSR marker. The serial numbers of the codominant SSR marker closely linked with the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK in the tobacco mainstream smoke are TMy80381 and IDYK04089, and the nucleotide sequences of PCR (Polymerase Chain Reaction) amplification products of the codominant SSR marker closely linked with the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK in the tobacco mainstream smoke are respectively shown as SEQ ID No. 1, SEQ ID No. 2, SEQ ID No. 3 and SEQ ID No. 4. The application refers to the application of the codominant SSR marker closely linked with the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK in detecting whether the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK exists in a tobacco genome DNA (Deoxyribose Nucleic Acid) or not, and the application refers to the application of the codominant SSR marker closely linked with the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK. Compared with a method in the prior art, the codominant SSR marker disclosed by the invention has the characteristics of accuracy, high efficiency, stability, convenience, low cost and no detection opportunity limitation, so that the molecular marker can be applied to molecular marker-assisted selection of the 4-methyl nitrosamine-1-3-pyridyl-1-butanone release amount gene qNNK in low-harm tobacco breeding.