The spatial distribution pattern of mixed populations of white grubs was aggregated at the late growing stage of Paeonia Lactiflora in the field. The further x~2-test indicates that 6 groups of experimental data all agree to negative binomial distribution. As a result, a model for theoretical sampling number is established, which differs with confidence degrees, insect population density and adoptable errors. Based on the sequential sampling method for negative binomial distribution, a sequential sampling model is concluded for the mixed white grub population at different population levels.