Abstract: To address issues such as slow filling speed and high cost in existing SF₆/N₂ gas-insulated switchgear (GIS) filling devices, a direct pipeline filling method based on heated-pipeline gas flow control technology is proposed. This approach eliminates the need for built-in mass flow controllers (MFCs). Through theoretical analysis and ANSYS simulations, experimental parameters for the gas flow–temperature rise test platform were determined. A 3-meter heated pipeline was constructed, and gas flow prediction models were established for initial temperature differences of 20 °C and 30 °C. Precise gas flow regulation was achieved using a cascade fuzzy PID controller. Experimental results show that the cascade fuzzy PID controller outperforms the cascade PID controller across all performance metrics. When the initial temperature difference for SF₆ is 20 °C or 30 °C and 30 °C for N₂, the deviation in the SF₆ proportion in the mixed gas remains below 1.0%. A prototype filling device was designed and tested, demonstrating that when the filling pressure exceeds 0.2 MPa, the SF₆ proportion deviation complies with the standard DL/T 2243—2021, meeting on-site filling requirements.