The invention discloses a hydraulic automatic wire sizing and shearing mechanism. The mechanism comprises a shearing cutter, a ruler rod, a switch valve pulling plate and a hydraulic switch device, wherein the shearing cutter is driven to act through a shearing oil cylinder; the hydraulic switch device, comprising a switch valve shaft for controlling the oil channel to be opened or closed, is arranged on an oil channel of the shearing oil cylinder; the switch valve pull plate is connected to the switch valve shaft of the hydraulic switch device; a sizing impacting block which is positioned on a wire conveying way is mounted on the ruler rod and is connected to a synchronous rotating sizing shifting block through a sizing rotating shaft; the sizing shifting block is connected to the switch valve pulling plate. The hydraulic automatic wire sizing and shearing mechanism is simple in structure; all actions are in close fitting and connection; the length of a reinforced steel bar can be accurately measured; the reinforced steel bar can be automatically sheared; the mechanical operation replaces the manual operation, so that the production efficiency and the shearing accuracy can be improved.