Abstract: For heavy-ion radiotherapy, HIRFL-CSR (Heavy Ion Research Facility in Lanzhou-Cooler Storage Ring) needs a long term uniform ion beam extraction from HIRFL-CSR main ring to high energy beam transport line to meet the requirement of heavy-ion radiotherapys ion beam. Slow extraction control system uses the synchronous signal of HIRFL-CSR control systems timing system to realize process control. When the synchronous event data of HIRFL-CSR control systems timing system trigger controlling and changing data (frequency value, tune value, voltage value), the waveform generator will generate waveform by frequency value, tune value and voltage value, and will amplify the generated waveform by power amplifier to electrostatic deflector to achieve RF-KO slow extraction. The synchronous event receiver of slow extraction system is designed by using FPGA and optical fiber interface to keep high transmission speed and anti-jamming. HIRFL-CSRs running for heavy-ion radiotherapy and ten thousand seconds long period slow extraction experiments show that slow extraction control system is workable and can meet the requirement of heavy-ion radiotherapys ion beam.