The invention provides a swill-cooked dirty oil detecting system. A semi-reflection and semi-transmission beam splitter is arranged at an emission port of a femtosecond laser; a pulse laser light is split into a pump light path and a detecting light path; the pump light path is connected with a photochopper, a first attenuation sheet, a first reflecting mirror, a second reflecting mirror, a first focusing lens and a gallium arsenide photoconductive antenna in sequence; a sample box is arranged on a confocal point of second and third paraboloidal mirrors; a second collimating lens, a half wave plate, a light path delay device, a third reflecting mirror, a polarizer and a high resistivity silicon reflecting mirror are arranged on the detecting light path in sequence; the femtosecond laser after being sampled by an electrooptic crystal is detected by being directed with a photoelectric detector via a quarter wave plate, a third focusing lens, a fourth reflecting mirror, a fourth focusing lens, a Wollstone prism as well as second and third attenuation sheets; and the variation of voltage magnitude caused by terahertz waves is recorded, so that a time domain terahertz signal including swill-cooked dirty oil information is obtained. The swill-cooked dirty oil detecting system provided by the invention has the advantages of simple detecting process, high efficiency, rapidness and accurate detecting result.