Deraeocoris brevis Knight (Hemiptera: Miridae) is a generalist predator and is a key natural enemy in pear orchards in the northwestern United States. Although D. brevis undoubtedly contributes to the regulation of major pear pests, pesticides often disrupt its activity and reduce its effectiveness as a biological control agent. A temperature-dependent stage-structured matrix model was developed to analyse the population dynamics of D. brevis influenced by insecticides. In this study, impacts of acetamiprid on field populations of D. brevis were analysed. The age class of D. brevis was categorized into four stages: eggs, small nymphs (1st to 3rd instar), large nymphs (4th to 5th instar), and adults. Probabilities for each element in the projection matrix were estimated using published temperature-dependent developmental data of D. brevis. Transition probabilities from an age class to the next age class or the probabilities of remaining in the age class were obtained from development rate function of each stage (age classes). The fecundity coefficients of adult population were the products of adult longevity completion rate (1/longevity) and temperature-dependent total fecundity. Also, direct and indirect residual effects of acetamiprid were incorporated into the model. The model results were much overestimated compared with observed actual data from 25d after model running. Such a discrepancy might be occurred from various reasons such as an intra-species competition, successful fecundity rate, etc. Further, the improvement and application of the model were discussed.