The disclosure relates to mineral processing, and more particularly to a copper(II)-ammonia complex ion sulfidization activator, and its preparation and application. A molar ratio of NH3 to Cu2+ in the active ingredient of the copper(II)-ammonia complex ion sulfidization activator is 2:1-4:1. The preparation method includes: dropwise adding an ammonia solution to a copper salt solution; and adjusting the mixture to pH 6-7.2 with dilute sulfuric acid to obtain the copper(II)-ammonia complex ion sulfidization activator. During the sulfidization flotation for the copper oxide ore, the copper(II)-ammonia complex ion sulfidization activator is added and mixed uniformly with the ore slurry prior to the introduction of the sulfidizing agent.