A card reader auxiliary device and a working method thereof, which relate to the field of card readers. The card reader auxiliary device detects whether a card exists based on contactless field of the card reader, when detecting that the card exists, the card reader auxiliary device opens its own contactless field and provides field strength enough for a fingerprint card to enable the fingerprint card to complete fingerprint comparison, the card reader auxiliary device obtains a current time of a timer, determines whether the current time is more than a first preset time, if yes, the card reader auxiliary device closes its own contactless field. According to the embodiments, the fingerprint comparison can be done by a card without updating the card reader, which can make the card communicate smoothly with the card reader.