Abstract: A new method of defocus length detection based on optical modulator is presented after analyzing traditional detection methods. Using this method, the convergent beam is modulated with the piezoelectric ceramic driving deformable mirrors, and received by photoelectric detectors through a pinhole or slit. Then the defocus length can be obtained after correlation detection. The method can obtain defocus length and direction by one detector at the same time, and the defocus voltage varies linearly with defocus length. Experiments were conducted, in which the beam was modulated by a 100 Hz sine signal. Detected voltages within 4 mm before and 2 mm behind the focal plane were obtained, which are consistent with theoretical derivation. The technique can be applied to circular symmetric and non-c