The phytase-producing yeast RCEF4907 were identified as Issatchenkia orientalis based on its sequence,the morphological and physiological characteristics.A large number of mutants were qualitatively screened by calcium phytate agar medium after I.orientalis RCEF4907 isolate was mutated with Nitrosoguanidine,Ultraviolet,Nitrous acid and Lithium chloride.The ratio of clear zones to colony diameter of the mutants was used to estimate extracellular phytase production.The high-yield phytase mutant was isolated by quantitative analysis of phytase activity of fermentation broth with Vanadium-Ammonium Molybdate colorimetric method at 37℃ and pH2.5.The phytase activity of the mutant G3297 obtained from a great amount of mutants was 409U/mL,which was 5.76 times than the I.orientalis RCEF4907.