An optical device capable of effecting changeable beam angles, wherein the optical device comprises: a light source plate; a convergence lens; and at least two diffusion lenses (4, 5) comprising a first diffusion lens and at least one backup diffusion lens; wherein when one of the at least two diffusion lenses (4, 5) is in use, the rest of the at least two diffusion lenses (4, 5) are in a stand-by state; wherein the optical device is configured such that the light emitted by the light source plate is collimated by the convergence lens and allowed to go through the diffusion lens in use to be diffused; wherein the optical device is configured to cause the collimated light incident on refraction surfaces of the different diffusion lenses (4, 5) with different incident angles to exit from the different diffusion lenses (4, 5) with different emergence angles; wherein the optical device is capable of employing different diffusion lenses (4, 5) for changing the beam angle of the optical device; wherein each one of the at least two diffusion lenses (4, 5) comprises a first planar surface, a second surface, and a plurality of protrusions continuously formed on the second surface; the protrusions forming the refraction surfaces, characterized in that the optical device further comprises a glare suppressing ring for eliminating strayed light, wherein the glare suppressing ring has an opening extending from a front end to a back end along a light exiting direction, and a diameter of the opening increases gradually from the back end to the front end; one of the at least two diffusion lenses (4, 5) is arranged at the opening at the back end; wherein the glare suppressing ring comprises a pressing ring extending inwardly along a radial direction at the opening at the back end; each one of the at least two diffusion lenses (4, 5) has a planar circular ring (42, 52) at the periphery on the second surface; the pressing ring abuts against the planar circular ring (42, 52), thus fixing the diffusion lens in the opening at the back end of the glare suppressing ring; and further characterized in that: the first planar surface faces the convergence lens, the second surface faces away from the convergence lens the surfaces of the first diffusion lens and the backup diffusion lens facing the convergence lens are flat surfaces; the other surfaces of the first diffusion lens and the backup diffusion lens have a plurality of convex portions (41, 51) continuously formed; the convex portions (41, 51) form refraction surfaces; and the first diffusion lens and the backup diffusion lens have different protruding heights on their convex portions (41, 51). a light source plate; a convergence lens; and at least two diffusion lenses (4, 5) comprising a first diffusion lens and at least one backup diffusion lens; wherein when one of the at least two diffusion lenses (4, 5) is in use, the rest of the at least two diffusion lenses (4, 5) are in a stand-by state; wherein the optical device is configured such that the light emitted by the light source plate is collimated by the convergence lens and allowed to go through the diffusion lens in use to be diffused; wherein the optical device is configured to cause the collimated light incident on refraction surfaces of the different diffusion lenses (4, 5) with different incident angles to exit from the different diffusion lenses (4, 5) with different emergence angles; wherein the optical device is capable of employing different diffusion lenses (4, 5) for changing the beam angle of the optical device; wherein each one of the at least two diffusion lenses (4, 5) comprises a first planar surface, a second surface, and a plurality of protrusions continuously formed on the second surface; the protrusions forming the refraction surfaces, characterized in that the optical device further comprises a glare suppressing ring for eliminating strayed light, wherein the glare suppressing ring has an opening extending from a front end to a back end along a light exiting direction, and a diameter of the opening increases gradually from the back end to the front end; one of the at least two diffusion lenses (4, 5) is arranged at the opening at the back end; wherein the glare suppressing ring comprises a pressing ring extending inwardly along a radial direction at the opening at the back end; each one of the at least two diffusion lenses (4, 5) has a planar circular ring (42, 52) at the periphery on the second surface; the pressing ring abuts against the planar circular ring (42, 52), thus fixing the diffusion lens in the opening at the back end of the glare suppressing ring; and further characterized in that: the first planar surface faces the convergence lens, the second surface faces away from the convergence lens the surfaces of the first diffusion lens and the backup diffusion lens facing the convergence lens are flat surfaces; the other surfaces of the first diffusion lens and the backup diffusion lens have a plurality of convex portions (41, 51) continuously formed; the convex portions (41, 51) form refraction surfaces; and the first diffusion lens and the backup diffusion lens have different protruding heights on their convex portions (41, 51).