Abstract: Offshore oilfields are developed mostly by virtue of horizontal wells and highly deviated wells, but the development wells have multiple layers with great interlayer contradictions, so it is necessary to control the interlayer production performance in real time. The separate-layer downhole hydraulic control method has the advantages of high reliability and great operation force, but under the influence of the number of pipelines, it is limited in the number of operation layers. The downhole decoding technology reduces the number of multi-layer control pipelines and improves the applicability of the hydraulic control process. The newly developed downhole decoder adopts the structure of sliding value and realizes horizon identification and pressure fluid guide through different pipeline pressure sequences. It controls 6 downhole horizons through 3 pipelines, so as to meet the need of separate-layer fine control. The experiments show that under the designated pipeline connection order, the decoder can be opened only under the designated pressure sequence, so as to avoid interlayer interference. What’s more, the transmission time of hydraulic transmission oil is experimentally verified. It is shown that under the temperature of 20 ℃, it takes Shell Tellus No.22 hydraulic transmission oil (5 MPa) about 240 s to transmit to the end of hydraulic control pipeline (length 3 000 m and diameter 6.35 mm). The research results provide a theoretical base for the control of downhole hydraulic control sliding sleeve.