A long span cable-stayed space truss is a hybrid structural system with a denser frequency spectrum and longer periods. Here, the required mode numbers determined using modal mass contribution ratio to horizontal seismic response of two models of two spans with one tower and three spans with two towers for cable-stayed space truss and its horijontal senismic response distribution law were studied by using the mode superposition response spectrum method. Time history analyses were performed for two models. Results showed that the first 20 modes particioating earthquake responses have enough accuracy; the maximum vertical displacement of the space truss, horizontal displacement of tower top and internal forces of members of the space truss and the cables are not big, but internal forces of tower roots are relatively great. In process of design, roots of tower should be regarded; for the model of two spans with one tower, the results using the time history analysis method are bigger than those using the response spectrum method. Therefore, complementary calculations using the time history analysis procedure under multi-earthqyake should be performed for systems of two spans with one tower.