By means of state-space population model, the population dynamics and population control of Diamondback Moth(DBM) Plutella xylostella (L.) was simulated by adding the biological agents. In the vegetable field,by releasing Trichogramma( Trichogramma confusum Viggiani(= T. chilonis, Nagaraja,nec. Ishii), or spraying DBM granulosis virus(PxGV), or spraying Bacillus thuringiensis(B.t.), the indeces of population control( IIPC ) of DBM decreased in 0.2, 0.24, and 0.29 against the control respectively, but the index of population trend of DBM in control was as high as 29.3, the population of DBM still increased by control each one of the biological agents. In the field of spraying B.t. and releasing Trichogramma showed the complementary effect. In the field releasing Trichogramma, spraying PxGV and B.t. the population dynamics of DBM were simulated. The results showed that the DBM population were decreased in a low level even in the high index of population trend in control(table 2).