A card transaction device is provided. And it's disposed in a smart phone, the smart phone includes a transaction processing application, and the card transaction device includes a credit card reader, a converter, a microcontroller, a plug, and a data register. The credit card reader is configured to read and encode basic information of a card, so as to generate a first encoded data. The converter is configured to convert the input data into an audio signal. The microcontroller is configured to control and manage the card reader and the converter. The microcontroller can reset the card reader and the converter, the plug is configured to plug into an audio hole of the smart phone, and the audio signal is transmitted through the audio hole to the smart phone.