A general formula for computing the OTF of a defocused optical system is derived from the Kirchoff diffraction integration under an approximate condition which is weaker than the Fresnel condition. Abberations on Gaussian imaging plane and system defocussing are the parameters. The OTF s for a series of diffrently defocused imaging planes could be obtained by using this formula, avoiding a lot of abberation computing work for each actual imaging plane. Therefore this formula is very convenient for investigating the regularity of OTF of a dofooused optical system. In this paper a computational example is given.