The present application describes a control information transmission method, a terminal device, a network side device, and a communications system. The method may be implemented by a terminal device and include determining, based on a preset mapping relationship between bit combinations and modulation symbols, a modulation symbol corresponding to a to-be-transmitted bit combination and a resource used to send the determined modulation symbol, where in the mapping relationship, each bit combination corresponds to one modulation symbol to be sent on a first resource or one modulation symbol to be sent on a second resource. In bit combinations corresponding to modulation symbols sent on at least one of the first resource and the second resource, SR bits in at least two bit combinations are different, and ACK/NACK bits in at least two bit combinations are different. Furthermore, the method may include sending, on the determined resource, the modulation symbol corresponding to the to-be-transmitted bit combination.