Abstract: Under low flow forced circulation conditions, reverse flow may occur in some U-tubes. The effects of primary side inlet temperature, primary side total flow velocity, electric regulating valve resistance coefficient and secondary side mass flow rate on the critical point of the U-tube reverse flow under conditions of low flow forced circulation were studied by experiments. The results show that with the increase of the primary side inlet temperature, the critical flow velocity of the reverse flow increases. With the increase of primary side total flow velocity, the U-tube inlet temperature gradually increases during occurring reverse flow. From the experimental results, the electric regulating valve resistance coefficient has no significant effect on the critical point of reverse flow. With the increase of secondary side mass flow rate, the critical flow velocity of reverse flow slowly decreases.