The Mo-Co-MgO and W-Co-MgO catalysts were first prepared by the citric acid method. The single-walled carbon nanotubes(SWCNTs) were synthesized by decomposing CH4 in Ar over the Mo-Fe-MgO，Mo-Co-MgO and W-Co-MgO catalysts at 1123 K in the fluidized bed reactor. The effects of composition of the catalysts on the synthesis of SWCNTs were studied by Raman spectroscopy and transmission electron microscopy (TEM). And the mechanism of growth of SWCNTs was discussed. The results show that the citric acid method is effective for to preparing the catalysts and the high-quality SWCNTs were synthesized over the three catalysts. At 1123 K, the growth of SWCNTs over the catalysts may be similar to the liquid micro-area model. The dependence of diameter distribution on the composition of the catalysts was weak. There were some differences in the inner structure of SWCNTs synthesized over the three different catalysts. The carbon yield rates of products were increased by the addition of W and Mo in the catalysts.