This application discloses a method implemented at a server to facilitate secure offline transactions. The server receives, from a client device, an authorization request that includes a user identifier, first financial account information and a secure code. The server authenticates the authorization request, and sends a first transaction approval to the client device. Then, in accordance with the information received in the authorization request, the server facilitates a secure transaction between the client device and a point-of-sale (POS) machine while the client device is offline. Specifically, the server receives, from the POS machine, a transaction request that includes at least the user identifier and the security code. The server retrieves the first financial account information from a memory according to the user identifier and the security code, performs a transaction operation associated with the first financial account information, and sends a second transaction approval to the POS machine.