The invention discloses an online quenching oil filtering device for a thermal treatment furnace. The online quenching oil filtering device comprises a quenching oil groove, wherein the quenching oil groove is provided with an oil outlet and an oil return opening, and a first filter, an oil pump, a centrifugal machine, an oil storage groove and a second filter are sequentially connected between the oil outlet and the oil return opening through a pipeline. The online quenching oil filtering device can filter oil sludge and suspended particles in the quenching oil online in real time, so that the cooling performance of the quenching oil is guaranteed, the consumption of the quenching oil is reduced, and the thermal treatment effect of a product is improved.