A mobile communications device and a method for replying advertising message (ad) are provided. The method includes steps of: receiving an ad transmitted by the server; identifying whether the received ad is messages with a predetermined format; invoking a decoding program from a storage unit for decoding the received ad if the received ad is messages with the predetermined format; identifying the special identifiers to retrieve each part of the ad; displaying a basic content part and a reply content part of the ad after the ad has decoded; identifying the reply code the user inputs in code input area via the input unit; receiving a user's reply, generating a reply message includes a reply code corresponding to the user's reply, and sending the reply message.