Particles with electric charge 10-14 e in bulk mass are not excluded by present experiments. In the present letter we provide a feasible scheme to measure the millicharged particles via the optical cavity coupled to a levitated nanosphere. The results show that the optical probe spectrum of the nano-oscillator presents a tiny shift due to the existence of millicharged particles. Compare to the previous experiment the sensitivity can be improved by the using of a specific geometry to generate an electric field gradient and a pump-probe scheme to read the weak frequency shift. Owing to the very narrow linewidth(10-6 Hz) of the optical Kerr peak on the spectrum, this shift will be more obvious, which makes the millicharges more easy to be detectable. The technique proposed here paves the way for new applications for probing dark matter and nonzero charged neutrino in the condensed matter.