A method of advanced polymer chromatography(APC) was established for the determination of the molecular weights and their distributions of polyethylene glycol derivatives and the impurity contents, with monomethoxy polyethylene glycol propyl aldehyde(mPEG_pALD) as the representation for polyethylene glycol derivatives. The chromatographic conditions such as chromatographic column and mobile phase were optimized, while the effects of the concentration of sample(within the concentration range of 0.5-20mg/mL) and dissolution time were also investigated. The optimized chromatographic conditions were as follows: three ultra-high performance gel columns were connected in series at 40 ℃ with methanol-water(95∶5,by volume) as mobile phase at a flow rate of 0.5mL/min,and a refractive index detector was selected. The result showed that the weight-averaged molecular weight(Mw) of mPEG_pALD was 19444 and its distribution index(D) was 1.01,while the Mw for impurity 1 was 38703 with its D index of 1.01 and its content of 1.31%,and the Mw for impurity 2 was 61036 with its D index of 1.00 and its content of 0.70%.Compared with the traditional GPC method, this APC method could obtain the molecular weight and its distribution of mPEG_pALD with higher resolution and less time, and the content of impurity could also be acquired. The method provides a scientific basis for the research, development and quality control of PEG derivatives, and is applicable for the determination of molecular weight, molecular weight distribution and purity of other PEG derivatives.