A novel coaxial focusing method based on interference is proposed to compensate for the shortage of off-axis focusing methods in immersion lithography. The measurement light is projected on the wafer plane through a lithography objective, and then reflected by the wafer and entries the objective. The resulting image created by interference between the reference light and the measurement light is detected by the detection system, so the defocusing amount of the wafer is modulated in the phase of the interference image. The defocusing amount is obtained by demodulating the phase. The simulated experiment shows that the proposed method can achieve a focusing accuracy of λ/25 (λ=632.8 nm), and has good noise immunity, meeting the demand of high-precision, real-time as well as non-contact measurement in immersion lithography.