It has been proved that there exist asymmetric holographic spatial optical solit on pairs in biased photorefractive crystals. The two solitons from such a pair h ave equal width and unequal amplitudes. The soliton existence curves of such sol iton pairs are given and three regions of different behaviors exist in these curves. That is, for a given value of bias field, the width of holographic solitons depends strongly on the ratio of the total peak input intensity of the two solitons from a pair to the intensity of the background light when the ratio is small or large enough, whereas it remains relatively constant in a middle r egion. The extent of the middle region for the asymmetric case is smaller than t hat for the symmetric case.