Frequency-search method was presented bases that diluting excess gas by increasing fan frequency. The required airflow was calculated to dilute gas by the dilution formula, then from fan performance curve database, the curve was selected to take in ventilation networks calculating program by search method continually. By comparing the calculated volume with required volume, one curve was chosen whose air volume was a bit lager but proximate than required volume at last. By using frequency converter to adjust fan performance curve to the one selected to dilute gas, the experimental model was set up to simulate the fan changing of frequencies to dilute gas, while gas concentration gauge. Experimental results show that the smaller the fan normal frequency, and the lager its dilution ability.Within dilution ability, gas reduce to safe value, and its volume-regulation ratio is close to 1. It proved that frequency-search method is a fast, accurate, steady and servo method, which produce good dilution result and utilize efficiency of air volume.