Abstract: The primary objective of this research was to explore the thermal conversion behavior of blended biomass pellet fuels, utilizing spent mushroom substrate (M1)—characterized by its high ash content and low calorific value—and bamboo sawdust (M2)—noted for its high calorific value and low ash content—as principal feedstocks. The investigation employed simultaneous thermal analysis techniques, specifically Thermogravimetric Analysis coupled with Differential Scanning Calorimetry (TG-DSC), to evaluate the combustion properties. To facilitate a systematic comparison, four distinct blending ratios were established: M3 (M1:M2 = 6:4), M4 (M1:M2 = 7:3), M5 (M1:M2 = 8:2), and M6 (M1:M2 = 9:1). These formulations were analyzed in terms of combustion performance, heat release characteristics, synergistic effects, and kinetic parameters. The experimental results revealed that the incorporation of bamboo sawdust significantly reduced the residual ash content after combustion. Specifically, the residue levels of M3–M6 ranged from 26.63% to 36.90%, markedly lower than the 37.22% observed for pure M1. In terms of the comprehensive combustion characteristic index (S), the descending order was as follows: M2 (2.12) > M4 (1.64) > M3 (1.59) > M1 (1.41) > M5 (1.29) > M6 (1.22), suggesting that increasing the proportion of bamboo sawdust enhances combustion efficiency. DSC analysis further demonstrated that the heat release capacity of the mixed fuels improved with higher bamboo sawdust content, with values ranging from M2 (16332 J/g) down to M1 (13022 J/g), in the following sequence: M2 > M3 > M4 > M5 > M6 > M1. Additionally, synergy effect analysis indicated that M4 exhibited the most pronounced synergistic interaction during combustion, highlighting its superior combustion compatibility. Kinetic studies revealed that the activation energy increased as the bamboo sawdust proportion decreased from M3 to M6. Moreover, the pre-exponential factor data suggested that M3 possessed the fastest reaction rate, followed closely by M4. Taking into account combustion performance, synergistic effects, and raw material costs, M4 (7:3) emerged as the optimal blending ratio. It demonstrated a high calorific value (15270 J/g), minimal residue (28.23%), and efficient combustion characteristics, thereby offering a solid theoretical foundation and technical reference for the pelletization and utilization of agricultural and forestry waste.