The threshold energies of radial flow in reactions of 124Sn+124Sn and 124Ba+124Ba in central collisions are investigated within an isospin dependent quantum molecular dynamics model by using three different forms of symmetry energy. It is found that the neutron-rich system has smaller threshold energy of radial flow and its value depends strongly on the form of symmetry energy. It indicates that the threshold energy of radial flow may provide a new method to determine the symmetry energy of nuclear matter.