The utility model discloses a pretreatment device for fresh water, comprising a first-stage filter reaction device, an ozone-contacting reaction device, a biological activated carbon reaction device and a second-stage filter reaction device which are connected in sequence by pipelines, wherein a water sprayer is arranged on a pipeline between the first-stage filter reaction device and the ozone-contacting reaction device, and connected with an ozone generator. In the utility model, the pretreatment device has high processing efficiency, and the processed outlet water is stable and up to the standard, good raw water quality conditions are settled for a subsequent sewage-recycling project of a refining enterprise, and the investment and operating cost are lowered; all parts adopted by the pretreatment device are all existing mature products sold on markets, the processed outlet water can meet the index and requirement of the fresh water of the refining enterprise after all the parts are assembled for use, simultaneously, a control device is matched for use, therefore, the pretreatment device has the advantages of being convenient in operation and low in cost.