An automatic oil-water separation measurement and control system for free water dehydrators in joint stations relates to an oil-water separation measurement device and a measurement control method for free water dehydrators used in oil fields to solve the problems of difficult operation, poor accuracy and low reliability in liquid level measurement. A liquid level measurement module transmits signals to a main control module by a liquid level signal modulation module, an interface switching module and a USB interface module; the main control module transmits control signals to a valve open-close driving module by the USB interface module and the interface switching module. The method comprises the following steps of: acquiring conductivity of all detecting heads; computing the liquid level height by means of singularity detection; closing the oil inlet pipe and opening the oil discharge pipe according to the liquid level height value; closing the oil discharge pipe and opening the water discharge pipe according to the air-oil surface height value; closing the water discharge pipe and opening the liquid discharge pipe of the transition layer according to the transition layer-water layer height value; and opening the oil discharge pipe according to the oil-transition layer surface height value. The device and the method of the invention have the advantages of high accuracy and reliability.