According to the principle of fluorescent whitening agent, the whitening effect of fluorescent whitening agent on fiber or fabric mainly depends on the content of ultraviolet ray in sunlight and the concentration of fluorescent whitening agent on fabric. When the concentration of whitening agent reaches an appropriate concentration, the whitening effect is the best. If the whitening agent concentration exceeds a certain range, it will exceed the yellowing point of whitening agent. Far from improving the whitening effect of the fibers or fabrics, the whitening effect of the fibers or fabrics will decrease, i.e. yellowing of the fibers or fabrics.