Abstract: Polyether polyols are the main materials for polyurethane products. The characteristics of the polyurethane products are affected directly by the average molecular weight and its distribution of polyether polyols. Polyether polyols were analyzed by electrospray ionization spectrometry (ESI-MS). Mass spectrums of their molecular weight distribution were obtained. The charge state of ions to the peaks was determined according to the different mass-to-charge ratio between the peaks of the isotope. The intensity of the peak represents the concentration of correspond polyether polyol molecule. Accurate average molecular weight values of M_n, M_w, and the distribution values (PD) were obtained by calculation. While most of the mass-to-charge ratio differs in 58 u, it can be concluded that the repeat unit of the polyether polyols is propylene oxide. The method is simple, quick, and accurate with high resolution and sensitivity.