A settling tank with a built-in H-shaped pipeline is applied to separation of oil and water in produced fluid from oil wells of oilfields. A center tube is perpendicularly welded in the center of the bottom of a tank body; the upper end of the center tube penetrates through the top of the tank; a water feeding opening is formed in the lower part of the center tube; a side vertical tube is arranged on one side of the center tube and is parallel to the center tube; a transverse tube is welded between the center tube and the side vertical tube; the center tube is communicated with the side vertical tube by the transverse tube; the upper end of the side vertical tube penetrates through the top of the tank; a flange is fixed on the top of the side vertical tube; a breathing hole is formed in the center of the flange; and the lower part of the side vertical tube is communicated with a drain pipe. The settling tank can preliminarily separate oil from water, the separated water is drained smoothly, an oil-water interface in the tank is relatively stable, and a using effect is good.