Co-pyrolysis of biomass and plastic is an effective way of solid waste resource utilization, which currently faces the problem of unclear distribution patterns of pyrolysis products. This study of TGA and Py-GC/MS diagnostics is used to examine the characteristics of pyrolysis behavior, distribution of pyrolysis products, and related synergistic effects of cross mixing of different kinds of forestry and agricultural residues with plastic wastes. The study provides a scientific basis on the distribution of pyrolysis products in the subsequent directed product distribution. The TGA results indicate that in the co-pyrolysis process, the pyrolysis of biomass and plastic is relatively independent. The co-pyrolysis can help improve the performance and product distribution. The results obtained from Py-GC/MS indicate that the type of plastic provides an influence on the distribution of pyrolysis products; the synergistic effect varies that depends upon the type of plastic examined. The co-pyrolysis of plastics and biomass helps to further improve the quality of pyrolysis products. The response surface methodology results indicate that pyrolysis temperature, mixture ratio, and pyrolysis reaction time all provide certain influence on the pyrolysis products. The intensity of the influence is ranked from high to low in the order: pyrolysis temperature, mixture ratio, and pyrolysis reaction time. The research results provide a feasible approach for effectively utilizing residual agricultural waste and type of waste plastics for improving the product quality and amount.