Abstract: As the installation foundation of magnet support, the plinth is critical for the stability of the whole support unit. Different fabrication and installation processes of plinths make significant differences in performance of the support unit. Based on the stringent requirements of advanced synchrotron light source for the mechanical stability of magnets, the plinth installation pouring experiment was carried out. Several concrete construction techniques commonly applied in practice were tested with the plinths of uniform shape and size. The natural frequency of each plinth was measured by hammering method and its stability was evaluated. It is concluded from the test result that secondary grouting can effectively improve the stability of the plinth and the plinth of epoxy-based grouting shows better performance in stability than that of cement-based grouting. Based on the natural frequency test result, the influence of the plinth installation on the system mode is analyzed with the support system of High Energy Photon Source as an example. The experimental results have certain reference value for the plinth design of similar machines.