The mutal solubility and tie-line data were investigated at 25℃ under atomspheric pressure for ternary system of phosphoric acid （solute）, water (dilute) and fusel oil (solvent) consisting of 0.35 wt% ethyl alcohol, 0.45 wt% n-butyl, 0.31 wt% n-propyl alcohol, 0.33 wt% n-hexyl alcohol, 5.52 wt% iso-butyl alcohol, 93.04 wt% iso-amyl alcohol and other in trace. It was found the plait point occurred at the composition of 20.5 wt% water 58.9 wt% phosphoric acid and 20.6 wt% fusel oil. The fusel oil was a more effective solvent for extracting phosphoric acid than n-butyl alcohol or butyl acetate.