Abstract: Co-gasification of waste plastics and coal mixture becomes a promising way for the elimination of waste plastics. This study deals with the numerical simulation of the co-gasification of PET and coal in a fluidized bed reactor. The variations of both particle size and its density are taken into account, and the devolatilization reaction rate is also modified with the aim to consider the effect of particle inner temperature gradient. The effectiveness of the current model has been justified by comparing the simulation results with the available experimental data. In addition, the effects of inlet gas velocity, solid feed position, and initial particle size on the hydrodynamics and co-gasification reaction are discussed in detail. The results show that the conversion of volatiles leads to the formation of syngas and ultimately causes an intense fluctuation in the bed. A higher solid feed position induces the weak bed fluctuation and non-uniform gas temperature distribution. Furthermore, the larger initial size of combustible particle deteriorates the interphase heat transfer and postpones the generation of syngas. The particle shrinkage and the inner temperature gradient strongly affect the hydrodynamics and devolatilization reaction rate, thus, it is important to consider such factors in the simulations of gasification process.