Abstract: 【Objective】This research aimed to clarify the optimal concentration and time of ethylmethylsulfone(EMS) mutation treatment of flowering Chinese cabbage, it provided support for the innovation and utilization of EMS mutagenic technology in flowering Chinese cabbage germplasm resources and enriching the selection of flowering Chinese cabbage germplasm resources.【Method】Two high-generation inbred lines C40 and Xiao 80 Days were mutated by EMS solution of different concentrations(0.2%, 0.4%, 0.6% and 0.8%) under different treatment times(8, 12 and 16 h). The adult plant rate and rate of seed-setting plant after mutation were investigated to determine the best mutagenesis conditions.【Result】Under the same EMS treatment time, the adult plant rate and seed-setting plant rate of flowering Chinese cabbage decreased gradually with the increase of EMS concentration. When the EMS concentration was 0.6%, the adult plant rate of C40 and Xiao 80 Days were 65% and 70%, respectively;and the seed-setting plant rate of two materials were 13% and 31% respectively. When EMS concentration was 0.8%, the adult plant rate of C40 decreased to 50%, and the seed-setting plant rate of both materials was less than 20%. Under the same concentration of EMS, the longer the mutagenesis time was, the worse the adult plant rate and seed-setting plant rate. When EMS concentration was 0.4%, mutagenesis time was 12 h, the adult plant rates of C40 and Xiao 80 Days were both 75%, and the the seed-setting plant rate of two materials were 30% and 45% respectively. After 16 h, the adult plant rates were reduced to 60% and 55%, and the seed-setting plant rates were reduced, both below 20%. EMS treatment not only inhibited the adult plant rate, but also seriously affected the seed-setting plant rate. According to the adult plant rate and the seed-setting plant rate of M₁ population, and to ensure the maximum mutation efficiency and mutant population, the optimal EMS mutatgenic condition was determined as 0.4% EMS mutation treatment for 12 h. The tolerance of different genotypes of flowering Chinese cabbage to EMS was different, and the treatment time or concentration of EMS could be appropriately increased or decreased on plants grew well. 3000 seeds of Xiao 80 Days were treated with the best mutagenic conditions, and 500 M₁ mutant lines were obtained. In M₁ population, there were some variation characters such as yellowing, albinism, shrinkage, dwarfing and chimerism. 176 M₁ families with large seed quantity were randomly selected to construct M₂ population with 2110 individuals. 94 mutant individuals were found in M₂ population, and the total mutation frequency was 4.3%. There were 61 plants with leaf color or shape mutation, 31 plants with plant type mutation and 2 plants with flower color mutation.【Conclusion】EMS mutagenesis of flowering Chinese cabbage seed has obvious effect. The EMS mutant library is rich in phenotypic variation, especially the beneficial mutation of compact plant type and dark green leaf color, which can be used in the functional genomics research and breeding of flowering Chinese cabbage.