The present application relates to the field of communications, and disclosed are a DCI transmission method and device under cross-band carrier aggregation. The method is: for LTE-A UE which aggregates carriers having different TDD uplink/downlink configurations, according to a pre-arranged rule or high-layer signaling configuration information, determining whether a DAI and a UL index are contained in a DCI format which is used by a PDCCH for scheduling a PUSCH, and determining a CSS and a USS respectively and separately. Accordingly, it can be ensured that the size of a DCI which is transmitted in a CSS does not change, thus ensuring the normal transmission of common cell information, and at the same time, in a USS, according to the actual needs, a specific bit field in the DCI can be determined rationally, thus supporting the UL multi-frame scheduling and/or the resource overhead control of an ACK/NACK over a PUSCH.