Conventional pyrolysis biochar generally has underdeveloped pore structure, low surface activity and poor electrochemical performance, which seriously limit its application in supercapacitors. Herein, biochar was fabricated through KHCO₃ activation coupling microwave-assisted phosphoric acid hydrothermal carbonization (MPHTC). The results revealed that both MPHTC and KHCO₃ activation can further improve the physicochemical structure and electrochemical characteristics of crop straw-derived biochar. Biochar obtained by KHCO₃ activation coupling MPHTC (HBCK) exhibited the highest surface oxygen content and the most adsorbed O₂/H₂O and aromatic CO, as well as a larger proportion of defect structures. Moreover, CS-HBCK displayed advantageous pore structure, with the highest specific surface area (1058.36 cm²/g), the largest total pore volume (0.79 cm³/g), and a superior micropore proportion of 59.21 %. Hence, CS-HBCK exhibited the highest specific capacitance (317.01 F/g) with better rate performance and lower resistance, whose capacity retention rate could reach 87.33 % after 10,000 cycles of charge-discharge. Furthermore, the CS-HBCK symmetric capacitor could achieve an excellent energy density of 14.82 Wh/kg at the power density of 225 W/kg, whose coulombic efficiency could still be maintained at a high level (98.38 %) after 10,000 cycles of charge-discharge, indicating a great potential of recycled waste crop straw in energy storage systems.