The progress of N-alkylation reactions of poly (ethylene oxide) polyurethane conducting through a two-step reaction was monitored by In situ real-time infrared spectroscopy (In situ IR). Firstly, the starting polymer was metalalized with alkali metal catalyst and then the obtained urethane polyanion was treated with alkyl tosylate in DMSO/THF. In the case of 4,4-diphenylmethane diisocyanate (MDI) -based PEOPU, the absorption IR band of carbonyl at 1725 ㎝^(-1) (C=O) was shifted to 1628 ㎝^(-1) in the first step, which is due to the change of the electron environment from the formation of the anionic sites on urethane group. In second step, the band at 1628 ㎝^(-1) was shifted to 1702 ㎝^(-1), which is due to the disappearance of the anion and the formation of the N-alkyl urethane. A chemical conversion profile for post-polymerization N-alkylation reaction of PEOPU can be obtained from the monitoring of the change of the characteristic peak urethane carbonyl.