The invention discloses a fresh air handling unit with integrated functions of dehumidification and evaporative cooling, which comprises a first dehumidifying and cooling module, a constant-humidity cooling module, an evaporative cooling module, a liquor regenerating module, a heat source heat exchanger, a liquor regeneration circulating pump and a cold source heat exchanger. The first dehumidifying and cooling module is used for dehumidifying and cooling fresh air, the constant-humidity cooling module is used for realizing constant-humidity cooling for the fresh air, and the evaporative cooling module is used for realizing sufficient heat and moisture exchange of the fresh air and water. The invention additionally aims to disclose an air treatment method for the fresh air handling unit with the integrated functions of dehumidification and evaporative cooling. By the aid of the fresh air handling unit and the air treatment method, problems that performance coefficients of an existing unit in the prior art are low, the quality of indoor air is poor, noise is loud, a wind blowing feeling is strong, energy consumption of a fan is high and the like are solved. In addition, the fresh air handling unit and the air treatment method have the advantages that performance coefficients of the unit can be greatly increased, the fresh air handling unit and the air treatment method are widely applicable to high-temperature and high-humidity regions, excessive noise and wind blowing feeing can be avoided, energy consumption of a fan can be reduced, air can be sterilized, dust in the air can be removed, energy utilization rate can be improved, and the like.