A lamp holder for an LED lamp string includes a base and an LED luminaire. The LED luminaire is inserted into the base. The base includes a casing, a cover, and a tail plug which are axially symmetric structures. The combination of the casing, cover and tail plug form a pin insertion hole of the LED luminaire and a wire inserting hole of a connecting wire of the lamp string, and the three are combined as a whole by a latch member, a latch slot, and a latch member. The cover and the tail plug seal an electrically conductive portion of the LED luminaire to provide a waterproof and dustproof effect. Since the casing, cover and tail plug are bilaterally symmetric structures, the level of difficulty of the alignment is reduced significantly. Automated equipment may be used for assembling the base in mass production, and the labor is reduced significantly.