Focusing technique is the key factor in improving the resolution of projection lithography. In order to achieve high accuracy of focusing on the nanometer level, we present a focusing method based on dual-channel light intensity modulation. Two superposed grating fringes are formed with a phase difference of pi/2. The shift of the wafer can be resolved by the phase variation of corresponding fringes. The conventional focusing method with light intensity modulation is influenced by disturbing effects such as fluctuations of the light intensity and stray light. These adverse factors are overcome by the signal ratio of two channels with the same optical components. The focusing accuracy in our experiments is +/- 8 nm.