A heat-dissipating downlight lamp holder has a lamp holding unit, multiple fins and a bottom board. The fins are mounted on a periphery of the lamp holding unit to effectively dissipate heat of a light bulb mounted in the lamp holding unit out of the lamp holder. The bottom board located underneath the lamp holding unit has multiple air inlets formed through the bottom board to let cold air under the lamp holder enter the lamp holding unit through the air inlets. By convection, heat dissipated from the fins is carried away from the downlight, thereby effectively reducing heat accumulated by the light bulb and preventing the light bulb from being damaged by high heat.