Method and device used for wireless communications, including receiving a second media access control packet data unit (MAC PDU) group, a MAC Header of any MAC PDU in the second MAC PDU group comprising information corresponding to a first portion of a first old identifier (ID); the second MAC PDU group comprising a first packet data convergence protocol (PDCP) PDU, and a header of the first PDCP PDU comprising a first key ID; the first key ID being used to identify a first key, and the first key being used to generate a key for a security algorithm applied to the first PDCP PDU; transmitting a first MAC PDU group comprising a second PDCP PDU, the second PDCP PDU comprising information corresponding to a first portion of the first PDCP PDU; by determining a first key ID and a second key ID, reliability is improved, and risks during communications are avoided.