Copper azide (CA) is a promising energetic material with characteristics of high energy output and environmental friendliness that can be used as a microinitiator charge, but the extreme sensitivities hinder its practical applications. In situ preparation of CA can avoiding operating on sensitive CA directly and depressed the risk of handling. However, it is still a challenge to develop a straightforward, high-efficiency in situ preparation method. Consequently, a simple and rapid liquid-solid reaction strategy for azidation has been proposed via the in situ transformation of Cu(OH)2 into CA in a hydrazoic acid aqueous solution. In situ preparation of CA on a Cu film was performed to demonstrate the application of this strategy in detail. The transformation was completed in 4 min, which significantly improved the efficiency for production of CA compared to previously reported methods. This work provides a facile and highly efficient method for the in situ preparation of CA.