NaYbF4 nanocrystals were synthesized by thermal decomposition method and the size of nanocrystals were characterized by TEM images. The relationship between the concentration of NaYbF4 nanocrystals and their absorbances was established by the Beer-Lambert law. A method for adjusting the concentration of NaYbF4 nanocrystals was proposed by the absorption spectrum of Yb3+ in the NaYbF4 nanocrystal. And then the formula describing concentration adjustment was introduced. Furthermore, the method was tested by five batches of NaYbF4 nanocrystals with different concentrations, which show the method is very credible. And then the concentration of NaYbF4 nanocrystals after adjustment were measured several times to ensure the error of the adjustment method, the maximum error is 9%.