The control suitable occasion of the diamondback moth (DBM) was studied in Guangdong province.By means of rotation composite design of quadratic regression ,two methematical models were constructed to study the influence of the DBM on yield and quality of false pakchoi.The results showed that the sensitizing phase of false pakchoi to DBM was in the 6 to 9 lesf-stage.The results in experiment of the DBM selectively laying eggs showed that the most eggs were found in the 4 to 5 leaf-stage of false pakchoi.Therefore,the 6-9 leaf of false pakchoi was considered as suitable occasion for controlling DBM.