In this study, a three-section simulated moving bed (SMB) with an open-loop design is used to fractionate polyethylene glycol (PEG) with different molecular weights. The purchased PEGs are mixed and separated by the open-loop SMB. A size-exclusion column with a pore size ranging from 10 to 100 nm, TOSOH GMPW 7.5 mm × 30 cm, is used to separate the mixtures. Based on the Triangle theory, the operating parameters of the SMB are determined and used to separate the three binary mixtures. The results show that the PEG mixtures with molecular weights of 400 and 8000, and those of 1500 and 20,000, are separable, yet those of 1500 and 3500 are difficult to separate by the selected column. The relative elution for molecular weights of 400 and 8000, and 1500 and 2000, is 1.49 and 1.54, respectively, resulting in easy separation. However, the difference in the elution volume for mixtures with molecular weights of 1500 and 3500 is so small that the operation condition is confined to a tiny area on the (m(2), m(3)) plane defined by the Triangle theory. This makes robust application of the SMB impossible. Fortunately, it is still possible to obtain pure raffinate with low recovery, but a pure extract is still not possible. It is concluded that the low selectivity of the binary mixture and the fluctuation of the operation result in the difficulty in separating the 1500 and 3000 molecular weight mixtures. This paper presents the operation procedures, including the selection of the column, the discovery of selectivity, the application of the Triangle theory and the experimental results, in order to illustrate how to apply an SMB to the fractionation of PEGs.