The invention discloses a biogas slurry COD on-line monitoring and safe utilization regulating system. The biogas slurry COD on-line monitoring and safe utilization regulating system comprises a COD on-line monitoring instrument, a central system control box, a biogas slurry pool, a water storage pool, an irrigation pipeline, a pump room, a frequency converter and a turbine stirrer, wherein the pump room is composed of a biogas slurry pump and a water pump, the joint of the biogas slurry pump and the irrigation pipeline is closer to the water outlet of the irrigation pipeline compared with the joint of the water pump and the irrigation pipeline, the central system control box receives and processes a signal of the COD on-line monitoring instrument, and directly controls the operation of the water pump, the frequency converter controls the operation of the biogas slurry pump, and the turbine stirrer is arranged at the position, close to the water outlet of the irrigation pipeline, in the irrigation pipeline. The invention further discloses a use method of the safe utilization system. According to the system and the method provided by the invention, the COD on-line monitoring instrument is adopted for carrying out real-time monitoring on the biogas slurry before irrigation, and the combined application ratio of the biogas slurry to water is guided according to the monitoring result, so that the operating control based on the COD application safety is realized, further, the growth safety of crops is guaranteed, and in addition, an effective reference basis is provided for scientifically and reasonably utilizing the biogas slurry.