A method for keeping an active state of a smart card until re-entering a field, comprising: sending a waiting time extension instruction by a smart card to an electronic device when the smart card is in an electromagnetic field of the electronic device and in an active state, the waiting time extension instruction comprising at least a preset waiting duration; the method being characterized by: starting by the smart card a timer of the smart card after the smart card receives a reply returned by the electronic device in response to the waiting time extension instruction, a timing duration of the timer of the smart card being the preset waiting duration; determining in response to detecting by the smart card that the smart card has left the field whether the timer of the smart card has timed out, keeping a current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out, and changing by the smart card from the active state to a leaving-field state when it is determined that the timer of the smart card has timed out, wherein the smart card is powered by a built-in power supply of the smart card when the smart card has left the field; and determining in response to detecting by the smart card that the smart card has re-entered the field whether the timer of the smart card has timed out, and keeping the current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out. sending a waiting time extension instruction by a smart card to an electronic device when the smart card is in an electromagnetic field of the electronic device and in an active state, the waiting time extension instruction comprising at least a preset waiting duration; the method being characterized by: starting by the smart card a timer of the smart card after the smart card receives a reply returned by the electronic device in response to the waiting time extension instruction, a timing duration of the timer of the smart card being the preset waiting duration; determining in response to detecting by the smart card that the smart card has left the field whether the timer of the smart card has timed out, keeping a current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out, and changing by the smart card from the active state to a leaving-field state when it is determined that the timer of the smart card has timed out, wherein the smart card is powered by a built-in power supply of the smart card when the smart card has left the field; and determining in response to detecting by the smart card that the smart card has re-entered the field whether the timer of the smart card has timed out, and keeping the current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out. starting by the smart card a timer of the smart card after the smart card receives a reply returned by the electronic device in response to the waiting time extension instruction, a timing duration of the timer of the smart card being the preset waiting duration; determining in response to detecting by the smart card that the smart card has left the field whether the timer of the smart card has timed out, keeping a current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out, and changing by the smart card from the active state to a leaving-field state when it is determined that the timer of the smart card has timed out, wherein the smart card is powered by a built-in power supply of the smart card when the smart card has left the field; and determining in response to detecting by the smart card that the smart card has re-entered the field whether the timer of the smart card has timed out, and keeping the current state of the smart card as the active state when it is determined that the timer of the smart card has not timed out.