The present application provides a multimedia data transmission method and device, wherein the method comprises: a first transmission device located in a first network obtaining a multimedia file descriptor and a multimedia file to be transmitted which have a correspondence, wherein the multimedia file descriptor and the multimedia file are stored in different memories; the first transmission device encoding content of the multimedia file descriptor and content of the multimedia file into a multimedia transmission file in a first format according to a predetermined rule; the first transmission device sending the multimedia transmission file to a second transmission located in a second network. With the present application, the problem that multimedia data cannot be transmitted synchronously during ferrying in related arts is solved and the security and confidentiality of multimedia data transmission is ensured while the multimedia data is transmitted synchronously.