In this study, we used the concentrations of 0.1%, 0.2% and 0.3% colchicine solution to treat the shoot tip growth points for 4 times when diploid Purple Tsaitai grows to cotyledon stage, respectively. The autotetraploid Purple Tsaitai was identified and screened. Then the nutritional qualities between diploid and tetraploid Purple Tsaitai were compared. The results are as follows: (1) using 0.2% colchicine solution to drip four times had the best treatment effect on Purple Tsaitai, and the induction rate of tetraploid Purple Tsaitai was 6.62%. (2) Morphologically, the leaves, flower clusters, flower organs, siliques and seeds of the tetraploid plants are significantly different in the greatness compared with diploid plants; Anatomically, the stomata became larger, but the density of stomata in tetraploid plants decreased. In addition, pollen of tetraploid plants showed rectangular, trapezoidal and other irregular shapes. In cytology, the chromosomes numbers of tetraploid and diploid plants were 40 and 20; Flow cytometry identification results showed that the fluorescence intensity of diploid plants was 956 725.15, while the tetraploid plants was 209 238 5.03, which was about twice diploid. (3) In the nutritional quality of plants, compared with diploid plants in nutritional quality, the total content of free amino acids and total chlorophyll in tetraploid significantly increase by 228.58% and 110.02%, respectively. However, the total contents of nitrate nitrogen, soluble protein, vitamin C, soluble sugar and cellulose in the tetraploid Purple Tsaitai significantly reduced by 48.99%, 43.20%, 45.81%, 44.50% and 59.97%, respectively. (4) Compared with diploid, tetraploid plants showed increasing in plant weight, leaf thickness, leaf width and petiole width, decreasing in plant expansion. The study found that the optimum concentration of colchicine to induce tetraploid purple Tsaitai was 0.2%. At the same time, tetraploid purple Tsaitai showed obvious high yield, but its overall nutritional quality was not higher than that of diploid plants.