Abstract: Hydraulically controlled intelligent well systems transmit control signals and power to the underground through thousands of meters of hydraulic control pipelines, and realize horizon selection in target layers and flow rate control. Transmitted downhole hydraulic control signals are affected by transmission media and slender transmission pipeline. Thus, the transmission speed, strength, and shapes of these signals are subject to attenuation and distortion, which are difficult to identify by the downhole equipment. To achieve sound control over the downhole actuator, this study discussed the influence of the signal transmission speed and wellbore temperature variation along the depth on the viscosity of transmission media. The following aspects were explored: the variation of pressure over time when the wellhead pressure propagated downward; the change in the shapes of ground control signals when they were transmitted underground; the transmission characteristics when hydraulic control signals and hydraulic power signals were applied simultaneously; the change in control pressure to open downhole sliding sleeves with or without resistance. In addition, upon consideration of the influence of different inner pipeline diameters, pressure applied methods, wellbore environments, and viscosity of hydraulic oil on the above transmission characteristics, several quantitative evaluation conclusions were drawn. For example, the hydraulic control pressure should be greater than 5 MPa, and the downhole transmission time for hydraulic signals in a deep-offshore well with a depth of 3 000 m was about 25 min. The research conclusions could provide a theoretical reference for downhole hydraulic control.