We report the theoretical investigation of the Z-scan technique for characterizing the instantaneous Kerr nonlinearity of an optically thin multiphoton absorber. Based on the Huygens–Fresnel diffraction integral method, we study the characteristics of the closed-aperture Z-scan traces. Most importantly, we present the analytical formulae of the closed-aperture Z-scan transmittance with simultaneous Kerr effect and n-photon absorption. Besides, we demonstrate that the determination of the third-order nonlinear refractive index in multiphoton absorbers becomes timesaving yet unambiguously by the presented theory.