Herein, a new peroxymonosulfate (PMS) activation system was proposed employing nonredox metal ions as Lewis acids (LA), which have been widely recognized to play important roles in many biological and chemical oxidations. With Co ions as model catalysts, it was found that oxidizing power of PMS was enhanced after binding weak LA such as Ca ions, leading to its easier reduction to active radicals and substantial enhancement of dye degradation. The promoting effect of Ca was also observed with other cobalt catalysts including CoFeO and CoO. The rate of PMS decomposition in Co+LA/PMS system was correlated with Lewis acidity; while in the presence of strong LA including La and Y, the dye degradation rate declined. The interactions of LA with PMS were characterized and the detailed mechanism was proposed. The present study provides the first example of the promoting effect of weak LA on PMS activation with cobalt based catalysts.