Abstract: Aftet treating by chemical derivation under complete same condition, child patients'urine,auxiliagry sample coming from child patients themself and added certain amount phenyl-pyruvic were measured quantitatively by GC/MS. Through this way, quantity of phenyl-pyruvic in child patient's urine could be evaluated. This method overcame the difficulty in determining amount due to difference derivative result of pyenyl-pyruvic in child patients'urine and in standard organic aicd mixture, and also successfully solves the problem of quantitative measurement of pyenyl-pyruvic in urine.