We report on the theoretical, numerical and experimental investigation of secure acoustic holography. This secure holography is achieved via an encoded hologram generated by a transducer array decoded by a transmissive acoustic meta-key. An unreadable image is received if directly using the encoded hologram, while the desired image can be obtained only with the aid of a decoding metastructure, i.e., a correctly decoding meta-key. The proposed meta-key consists of a series of unit cells, each of them is composed of a straight channel decorated with several opening panels. By adjusting the opening size of unit cells within the meta-key, its transmitted phase profile can be effectively controlled, hence, the desired decoding meta-key can be successfully constructed. The patterns of a butterfly, sunglasses and the letters of NJUST are chosen as examples to demonstrate the performance of the secure acoustic holography. Vulnerability tests of the meta-key further confirm the safety and reliability of secure acoustic holography. With the advantages of simple design, flexible functionality, and high quality for reconstructed images, our work opens a new avenue for acoustic holography and may find applications in secure acoustic communications based on holograms.