The present invention provides a method for sending and receiving control information, an apparatus and a communication system. The method for sending control information includes: setting respective control bits in a downlink control information (DCI) format to generate control information applied by a network side to a terminal, where the control bits include validity indication bits and transmission control bits, indication information indicating whether transmission control exercised by the network side on a transmission block is valid relative to the terminal is written into the validity indication bits, and the validity indication bits are located outside the transmission control bits; and sending the generated control information to the terminal. In the embodiments of the present invention, a network side device extends or changes functions of some control bits in the DCI format of the control information, so as to improve efficiency of using a resource overhead occupied by the control bits in the DCI format of the control information, and improve efficiency of using an overhead required for transmitting the control information.