Double metal cladding waveguide can couple the light into the guiding layer to inspire the ultrahigh order modes, the incident waves overlap with the reflected waves to form standing waves in the guiding layer, and then a series of enhanced power points which are considered to be optical trapping points are formed. Based on this optical property, aiming at the shortcomings of the general optical reactor, we had designed a novel liquid-core waveguide optical reactor. Aqueous Fe2+/Fe3+ salt solutions in the hollow-core acted as the guiding layer of the reactor. Two hours after the light had coupled into the guiding layer under the special optical field, the synthetic products had been prepared. Depending on some analysis toward the products, it is sure that they are crystalline magnetite γ-Fe_2O3.