A light beam adjusting device includes a housing having a reflector, a light device disposed within the reflector for generating light which may be reflected partially upwardly and partially downwardly by the reflector. The light reflected downwardly by the reflector is allowed to emit out through the housing. A shielding panel is disposed in the housing, and an actuating device may rotate the shielding panel to open and block the lower portion of the housing, and to selectively open and block the light reflected upwardly by the reflector. The light reflected upwardly by the reflector may form a high beam together with the downwardly reflected light when the housing has not been blocked by the shielding panel.