The present invention discloses a controlled thermal-neutron nuclear fusion method. Its method is that the nuclear fuel is introduced into a magnetically-confined ring-shaped magnetron, and under the conditions of real-time controlled high-temp., low pressure and non-ionization the nuclear fuel is preheated by use of external electromagnetic wave so as to make the nuclear fuel absorb the sufficient heat quantity (electromagnetic wave) and latent heat and make their extranuclear electrons progressively accelerate to 2.99792458X10 to the power 8m/s, so that the electrons are fallen into the atomic nucleus and smoothly changed into the neutron state matter from its vapor state, and finally the controlled thermal-neutron state nuclear fusion reaction can be implemented.