The invention provides a fresh air handling unit adjusting and controlling system suitable for a radiant air conditioner and a controlling method of the fresh air handling unit adjusting and controlling system. The fresh air handling unit adjusting and controlling system is used by cooperating with a radiant air conditioner system in an indoor space; a fresh air dehumidifying unit is adopted to bear the fresh air load, the humidity load and part of indoor sensible heat load; a fresh air handling unit can achieve the different functions of filtering, cooling, dehumidifying, auxiliary heating, ventilating and the like according to season requirements and the outdoor fresh air state; in addition, two air supply channels, that is, the main air channel and the bypass air channel are designed in a fresh air handling unit indoor unit, gating is conducted through a gating valve, and thus the fresh air handling unit indoor unit can operate in different working modes through free switching; and through control of an adjusting and controlling device over the operation modes, an auxiliary radiating system adjusts the air environment of the indoor space, the indoor environment comfort level and the system operation energy efficiency ratio can be effectively increased, and the problems that the fresh air conveying energy efficiency ratio in the transition season is insufficient, the room temperature and humidity are unreasonable, moisture condensates on the surface of a radiant panel, and energy sources are consumed excessively are solved.