The output characteristics of semiconductor lasers under high speed large sinusoidal modulation have been analyzed numerically by means of a system of multilongitudinal-mode rate equations. It is shown that with different modulation parameters, the output may contain a regular pulse series,exhibit spectral widening and spectral shift, and under some adequate modulation conditions, 2-or multi-subharmonic short laser pulse of less than 10 ps may appear, and may be of single mode. A qualitative interpretation is given to some of these phenomena.