An optical device for illuminating a 3D solder paste thickness checking machine comprises a plurality of red LED lamps, a plurality of white LED lamps, LED lamp panels, a plurality of aluminum profile shells, light evening plates and a light source fixing plate. The light source fixing plate is of a polygonal annular structure and comprises a bottom and lateral sides, wherein the included angle between the bottom of each lateral side is 150 degrees. Each aluminum profile shell is a rectangular groove, slide grooves are formed in the middle and the opening edges of the groove, the corresponding LED lamp panel is mounted in the slide groove in the middle, the corresponding LED lamp is mounted on the LED lamp panel, and the corresponding light evening plate is mounted in the slide grooves in the opening edges of the aluminum profile shell. The white LED lamps and the white LED lamps are alternately mounted in the aluminum profile shells fixed on the lateral sides of the light source fixing plate. By the optical device, the problem of uneven illuminating strength during operation of the 3D solder paste thickness checking machine is solved.