Camellia japonica is a famous ornamental plant, which has been seriously infested with the scale insects in recent years. The camellia plants with resistance and susceptibility to the scale insects were found in our previous research, and there were significant differences in genotype, leaf structure, lignin content, malondialdehyde (MDA) content, and antioxidant enzyme activity. The scale insects were identified as Pseudaulacaspis cockerelli by the method of morphological identification. The pentose and glutathione metabolism pathway, phenylpropanoid biosynthesis pathway, ascorbate, and aldarate metabolism pathway, and glutathione metabolism pathway were the main metabolic pathways which were the most significantly enriched for differentially expressed genes (DEGs) in RNA-Seq. Transcription factors such as MYBS3, ERF27, bHLH17, bZIP17, NAC90, and WRKY40 played active roles to the scale insects. The expression of the PME and PG gene were up-regulated and overexpression of CjPME28 or CjPG1 in tobacco plants significantly improved resistance to the scale insects and affected them to grow, which demonstrated their roles of resistance to the scale insects in the camellia plants. Collectively, in this study, a camellia resource with resistance to the scale insects was screened, and the mechanism of insect resistance of camellia plants was revealed which provided evidence for the identification of new varieties and also provided a new perspective for the in-depth study of the mechanism of insect resistance of ornamental plants.