Induced Seedlings and differentiated rank seedlings from the explant of the stem tip apex tissue culture of the Welsh onion were obtained directly. Inducing callus from basal plate, the inducing regeneration plant from callus. In this paper, the variation in chromosome ploidy level of Welsh onion during tissue culture was studied by counting chromosome number. The results showed that the number of chromosome, which was from the cultured seedlings of meristem tip and the rank seedlings of meristem tip, was not altered, both the number were 2n=16,the hereditary stability of callus and regeneration plant was poor; and the variation percentage of chromosome in callus was 43.4%, among which haploid accounted for 6.7%, triploid accounted for 2.5%,tetraploid accounted for 10%,pentaploid accounted for 4.2%,hexaploid accounted for 3.3%,septuploid accounted for 4.2%,octoploid accounted for 3.3%,dysploid accounted for9.2%;the chromosome variation percentage of differentiation seedlings derived from callus was11.7% ,among which haploid was 6.7%,triploid accounted for 1.7%,tetraploid was 3.3%.