The invention discloses a dirty oil water separator with a built-in flow restraining device; the dirty oil water separator comprises a separator body, the flow restraining device and a vacuum pump. The separator body is provided with an extraction port communicating with the vacuum pump and an input port for influx of dirty oil water, and the separator body is internally provided with a cylindrical first filter core arranged vertically and used for coarse filtration. The flow restraining device comprises an input part and at least two flow restraining parts which are connected with each other, the flow restraining parts are used for restraining the flowing-out direction of the dirty oil water, and output ports of the flow restraining parts are distributed along the consistent angle direction. The flow restraining device is assembled on the top side of the first filter core, and the output ports of the flow restraining parts are located between the first filter core and a shell body of the separator body. The flow restraining device is used for retaining the flow direction of the dirty oil water flowing to the first filter core, is used for oil and water pre-separation treatment of the dirty oil water and is used for timely removing dirty oil droplets gathered on the first filter core, so the filtration load of the filter core is reduced, oil-water separation is performed in vacuum conditions, and the separation effect and the separation efficiency of the separator are facilitated to be improved.