The decoherence of Rabi oscillation with multi-level processes in semiconductor quantum dots excited by laser pulses is investigated. By using population dynamic equations of multi-level system, the effect of three kinds of multi-level processes on the damping of Rabi oscillation in quantum dots are numerically simulated and discussed. The effect of biexciton can be neglected when the pulse width is larger than 5ps; the population leakage to wetting layer results in the decreasing of the amplitude and average of the population oscillation on the exciton ground state with the increasing of the excitation intensity; the effect of the two kinds of Auger capture processes on the Rabi oscillation and the spectral width of the photoluminescence from the exciton recombination are also discussed.