The invention discloses a pretreatment process and system for MVR evaporation of sewage. The pretreatment process sequentially comprises the steps of acidification, pre-decarburization, decarburization and neutralization, wherein pH value of sewage finally flowing into an MVR raw water tank is more than 6.5. According to the pretreatment process, by removing dirt causing factors, the scaling preventing effect is achieved; and only acid and alkali are used in the process, and no other reagent is added, so that secondary waste is not generated. Compared with a pretreatment manner for removing metals such as calcium and magnesium in a water body by virtue of dosing and coagulation, the pretreatment process has the advantages that a large number of sludge which is generated in dosing and coagulation treatment manners and cannot be dried by virtue of conventional measures is not generated, furthermore, the pretreatment cost is not increased, and the pretreatment process has relatively high reality applicability.