The optical absorption spectra of Si-2-Si-33 clusters were systematically studied by a time-dependent density functional theory approach. The calculations revealed that the absorption spectrum becomes significantly broad with increasing cluster size, stretching from ultraviolet to the infrared region. The absorption spectra are closely related to the structural motifs. With increasing cluster size, the absorption intensity of cage structures gradually increases, but the absorption curves of the prolate and the Y-shaped structures are very sensitive to cluster size. If the transition energy reaches similar to 12 eV, it is noted that all the clusters have remarkable absorption in deep ultraviolet region of 100-200 nm, and the maximum absorption intensity is similar to 100 times that in the visible region. Further, the optical responses to doping in the Si clusters were studied.