In the frame of time-dependent density functional theory, the dynamical polarizabilities of Na5, Na6 and Na7 clusters are calculated using a time-dependent local density approximation. By using Fourier transformation, the optical absorption spectra of Na5, Na6 and Na7 clusters are obtained from their dynamical polarizabilities. It is shown that experimentally measured optical absorption spectra of Na5, Na6 and Na7 clusters are reproduced in our calculations. Furthermore, the calculations of Na6 and Na7 clusters are in good agreement with the results of configuration interaction method. Compared with the three-dimensional structure of Na6, the calculated optical absorption spectra of Na6 with the two-dimensional structure are more close to the experimental data.