The invention discloses a radiation refrigeration fiber design method and a radiation refrigeration fiber. The radiation refrigeration fiber design method comprises the steps of 1, selecting micro-nano granular materials of different grain sizes within a preset range, and calculating scattering efficiency curves of the micro-nano granular materials of the different grain sizes; 2, forming multipleequivalent structures, and calculating reflectivity data, corresponding to a visible-near-infrared band, of the multiple equivalent structures; 3, according to a preset reflectivity formula, conducting fitting on the reflectivity data obtained in step 2, and conducting further extrapolation to obtain more reflectivity data; and 4, according to the reflectivity data obtained through extrapolationof step 3, calculating the weighting reflectivity, corresponding to the visible-near-infrared band, of all the equivalent structures under a preset solar spectrum, and according to the weighting reflectivity, obtaining optimal grain sizes, under different thicknesses, of micro-nano grains. By means of the design method, the optimal grain sizes of the filling nano grains are quickly determined forpreset materials within the preset fiber thickness range so that the highest solar spectrum reflection efficiency can be achieved under a preset volume percentage.