The variable of multisupport excitation effect, which is extracted from the analytical formulae of the power spectrum density function (PSDF) of seismic response of a 2mass with 2support simplified system, was proposed to study the variation of the random seismic response due to the wave passage effect and incoherence effect. And a numerical example of a gatetype truss structure was given to validate the practicability of the results gained from the simplified system. It is shown that the quasistatic response will increase while considering the wave passage effect or incoherence effect; the relative dynamic response fluctuates with the increase of wave passage frequency, and the extreme point's distribution is determined by the ratio of natural vibration frequency to wave passage frequency; and the incoherence effect will not change the extreme points' distribution, but reduce the fluctuation amplitude. For actual structures, more wave velocities are proposed to be employed to obtain reliable results, since they have a strong influence on the seismic response. The incoherence effect has little influence on the actual structures, especially when the structure's span is small.