A method for maintaining transmission control protocol (TCP) connection and a computer system are provided. The method is adapted to a computer system having a network card and a processor, wherein the processor executes an operating system. In the method, a first TCP connection with a remote server is first established via the network card. Then, after the operating system enters a power saving mode, a wake-up packet transmitted by the remote server through the first TCP connection is received to wake the operating system. The operating system resets the retransmission time of the remote server transmitting the wake-up packet when resuming a normal operation mode.