A hydrocarbon suspension bed hydrogenation method using combined dispersed catalysts with large and small particle sizes is suitable for a suspension bed hydrogenation thermal cracking reaction process R10 of vacuum residue R10F, the adopted dispersed catalysts comprise small particle size LP which is difficult to mechanically separate and large particle size BP which is easy to mechanically separate, the average particle size of the BP is usually 5 times higher than the average particle size of the LP, the LP provides a high dispersion and a large catalyst contact area, the BP provides a large single particle area to prevent heavy asphalt HWA from wrapping the catalyst particles and forming an excessive shielding effect, the BP carried by reaction products is mechanically separated, returns to the R10 and cyclically accumulates to maintain tens to hundreds of the particle number of the circulating catalysts and the external surface area of the catalysts, and an LP-containing residueproduct returns to the R10 and cyclically accumulates to greatly improve the hydrogenation thermal cracking reaction effect of the HWA, other colloidal asphaltenes and wax oil, reduce the total amount of the fresh catalysts and improve the overall economy of the process.