We propose an efficient direct k-domain interpolation based on spectral phase in swept-source optical coherence tomography (SS-OCT). Both the calibration signal from the Mach-Zehnder interferometer (MZI) and the OCT imaging signal from the Michelson interferometer sharing the same swept source are detected and digitized simultaneously. Sufficient sampling of the OCT imaging signal with uniform k interval are directly interpolated in the k-domain based on the spectral phase derived from MZI calibration signal. Depth profile is then obtained from Fourier transform of the k-domain interpolated data. In vivo imaging of human finger skin and nail fold are conducted. Reconstructed images corresponding to different calibration methods are evaluated for comparison. Experimental results demonstrate that improved imaging quality with enhanced resolution and signal-to-noise ratio is realized by the proposed method in contrast to the spectral phase based time-domain interpolation method as well as the intensity based calibration method.