Relative encrgies of every Li-like Si ions energy lcvel (n=2-7) and ab initio Si XII wavelengths below 400A involving transitions between 1s2nl2L, levels up to n=7 are calculated. Corresponding transition probabilitics and line strengths are given. The accuracy of wavelength calculation is bettor than 1% comparing with ex sting data. The calculated wavelengths of Si XII 5d-3p, 5f-3d, 6d-3p and 6f-3d transitions are in good agreement (within 0.1%) with the observed transition wavelengths at which soft X-ray lasers have been experimentally demonstrated. The Si XII 7d-3p and 7f-3d transmitions are of potential interest for X-ray lasers with shorter wavelengths towards the Water Window.