In this paper, a general method for calculating the axial resolution (δ_z) of optical coherence tomography (OCT) system based on the Wiener-Khinchin theorem was proposed. The power spectral density (PSD) distribution of a light source was subject to inverse Fourier transform to obtain the self-coherence function of the source, and the full width at half maximum of the function was employed to calculate the δ_z. The δ_z values of the OCT systems illuminated by the Gaussian and non-Gaussian spectrum sources were calculated by this method, respectively, and its validity for the above sources was verified by comparing the calculated values with the nominal values of products given by the manufacturers. Furthermore, an ultra-broadband white light source was taken as an example to form a non-Gaussian spectrum by edge filtering with two filters. An experimental setup was built to measure the δ_z, and the result was very close to the calculated one with the proposed method. In conclusion, the method proposed in this paper can be applied to the δ_z calculation of OCT systems illuminated by non-Gaussian spectrum sources, providing a basis for the parameter consideration and component selection during system design.