A method for switching communication connection modes, a communication system, a base station, a transmitter and a receiver are provided. The method includes the following steps. Firstly, when a base station or a core network entity determines that a communication mode between a transmitter and a receiver should be switched from a cellular communication mode to a device-to-device communication mode, the base station sends a control message to the transmitter. Next, the transmitter stops transmitting at least one first packet to the base station in response to the control message. Afterwards, the receiver receives at least one transmission-unfinished packet and a transmission-unfinished packet status related to the at least one transmission-unfinished packet forwarded by the base station.