An activated carbon/palladium-gallium (Pd—Ga) liquid alloy composite catalyst, including a support and an active component supported on the support. The support is acid washed activated carbon. The active component is Pd—Ga liquid alloy. In the present invention, the active component Pd—Ga, present in the form of liquid alloy, forms a self-protective oxide layer. This protects acetylene from secondary reactions on the surface of the catalyst, inhibits or reduces acetylene to deeply hydrogenate to form ethane, thereby increasing ethylene selectivity. The present invention further provides a preparation method of the catalyst, where the catalyst of the present invention is prepared by immersion. The preparation method is simple and easy to operate. When the activated carbon/Pd—Ga liquid alloy composite catalyst provided by the present invention is used for acetylene hydrogenation to prepare ethylene, conversion rate of acetylene is as high as 99.8%, while the ethylene selectivity is as high as 98.9%.