In spectral domain optical coherence tomographic system, group velocity dispersion of broadband light source reduces axial resolution and leads to the decrease of image resolution and contrast. The decline of axial resolution of spectral domain optical coherence tomography (SD-OCT) caused by dispersion is analyzed and a method based on spectrum correction to improve axial resolution of spectral domain optical coherence tomography is proposed. Spectrum coordinate is processed by second-order coefficients fitting. The coefficients are got by three-step threshold iteration. The axial depth is demodulated by the corrected spectrum. Spectral-domain optical coherence tomography experiments of single-layer thin glass and tomato epidermis tissue show that, compared with traditional method, the axial resolution is improved more than 10 μm and resolution of the two-dimensional tomogram is higher. It is generally applicative for different samples.