The 426 nm blue light frequency doubling output based on the pumping light corresponding to D2 line of cesium atom is realized by the semi-monolithic resonant cavity embedded with a periodically poled potassium titanyl phosphate (PPKTP) crystal. Relatively loose focusing is adopted in experiment, which obviously improves the thermal effect induced by the absorption of fundamental frequency light and frequency doubling light. Frequency doubling blue light with the power of 117.2 mW is obtained based on the mode-matched fundamental frequency light with the power of 305 mW, and the highest optical-optical conversion efficiency in the frequency doubling process is up to 42%. The power fluctuation of blue light with the power of 84.5 mW is 0.48% in about an hour. Results show that the proposed method can realize the stable output of frequency doubling light, and has wide application prospects in the field of quantum optics and the interaction between light and matter.