Biomass-based pyrolytic polygeneration system can commercialize all products (liquids, gases, and solids) generated during pyrolysis, while fast pyrolysis, gasification and carbonization, can only singly commercialize liquids, gases, and solids, respectively. To determine the optimum operational parameters for biomass pyrolytic polygeneration while using bamboo waste as the feedstock, the product characteristics were investigated over a temperature range of 250 to 950 °C. Meanwhile, details of the evolution of the char structure were analyzed to reveal the pyrolysis mechanism. Results showed that to increase the yield of char, the operational temperature should be at 350 °C; however, at this temperature, no inner pores were formed and a low quality char product was produced. Thus, the optimum operating temperature recommended for biomass pyrolytic polygeneration of bamboo waste was set to 550 °C. At the optimum temperature, the surface area of the char was 200 m2/g, the calorific value of gas was 14 MJ/m3, and the concentration of phenols in liquid reached the maximum level. A pyrolysis mechanism based on the evolution of the char structure was proposed. First, the ordered organic macrostructure in raw biomass was converted to a network-like structure consisting of a “3D network of benzene rings” during the “initial decomposition stage (<450°C)”, and this was followed by the “first reconstruction stage (450–550°C)” whereby the initial 3D network was converted to a “2D structure of fused rings”. Subsequently, with further increases in temperature, a “graphite microcrystalline structure” was formed during the “second condensation stage (>550°C)”. The results of this study are expected to be beneficial for the comprehensive utilization of bamboo waste and provide new insight into the pyrolysis mechanism.