In this paper, the rotational composite design of quadratic regression was used to study the effects and interactions of four factors (the number of parasitoids released ,the density of host eggs, the size of searching patch and the size of the crops) on parasitism of Trichogramma pretiosum to the eggs of the diamondback moth(DBM), Plutella xylostella in the field. It was showed that the number of parasitoids released , the size of the crops and the size of searching patch influenced parasitism significantly in turn. The density of host eggs had only minor effect on parasitism. Interaction between the number of parasitoids released and the size of the crops was the most dominant factor among interaction of different factors affecting parasitism of T. pretiosum. T. pretiosum had a strong searching capacity to the egg of DBM, but there existed interference between individuals. Based on the data a quadratic regression parasitism model of T. pretiosum to the eggs of DBM was constructed.