An assembling electronic device includes a plurality of functional modules, a characterized casing having a plurality of receiving cavities, and a terminal circuit. Each of said functional modules is shaped and sized to detachably receive in the respective receiving cavity so as to securely retain the functional module in the characterized casing in position. The terminal circuit is provided at the characterized casing to electrically connect with the terminals of the functional modules when the functional modules are detachably received in the receiving cavities respectively so as to form a complete circuit for the functional modules, such that each of the functional modules is replaceably mounted in the characterized casing for enhancing a user-personalizability.