The invention belongs to the technical field of preparation of Fischer Tropsch synthesis catalysts, and particularly discloses a Fischer Tropsch synthesis catalyst for preparing a heavy hydrocarbon product through synthesis gas and a preparation method of the catalyst.The method includes the steps of preparing nanometer silicon dioxide spheres of a mesoporous structure, wherein the granularity of the spheres is 20-50 nm; adding the nanometer silicon dioxide spheres when cobaltosic oxide particles are prepared to obtain a cobalt-based catalyst, wherein the specific surface area of the catalyst reaches up to 762.1 m<2>.g<-1>, the pore capacity reaches 1.81 cm<3>.g<-1>, dual-pore pore size distribution is achieved, and cobaltosic oxide particles are uniform in size and high in dispersity and evenly dispersed among the nanometer silicon dioxide spheres.The catalyst is beneficial to dispersion of reactants and products when used for Fischer Tropsch synthesis reaction and has high activity and stability, low methane selectivity and high heavy hydrocarbon selectivity, and the heavy hydrocarbon selectivity reaches up to 85.5% or above.