To improve the thermal stability of poly (lactic acid),poly (lactic acid-co-phenylalanine) was prepared by introducing the phenyl of phenylalanine to the side chain.3-phenylmethyl-2,5-morpholinedione (PMD) and lactide (LA) were synthesized using phenylalanine and lactic acid as raw materials,respectively. Subsequently,poly (lactic acid-co-phenylalanine),homopolymer of PMD (PPMD) and homopolymer of LA (PLA) were obtained by ring-opening polymerization of PMD and LA as monomer.The amount of phenyl had an effect on the thermal stability,so a quantitative analysis by infrared spectroscopy was established in the present paper. Two homopolymers were determined by infrared spectroscopy to find out two characteristic peaks at 1671.53 and 870.82cm^-1,which were selected for PPMD and PLA,respectively.Choosing the characteristic peaks and according to the Lambert-Beer law,which is the linear relationship between absorbance and concentration of an absorbing species,a standard working curve was calculated:y=0.05567x+0.1091 (r=0.9993),where x was the reciprocal of PPMD monomer molar concentration and y was the absorbance ratio of PLA to PPMD at their characteristic peaks.Collecting the absorbance of poly (lactic acid-co-phenylalanine) at two characteristic peaks,the contents of copolymer were calculation by the working curve and the results were similar to that of 1H NMR by calculation from the ratio of proton area of PMD and PLA.It was noted that the relative error was below 2%.It was also confirmed that the content of copolymer could be determined by infrared spectroscopy conveniently and quickly.Meanwhile,this method could be adopted to determine the other poly (lactic acid-co-amino acid) practicably and widely.