Abstract: The under-liquid foam extinguishing system is safe, reliable and easy to put out a fire with a low cost, its defect is leakage in the check valve. This problem can be solved by means of installing the rupture disk between the check valve and control valve, or mstalling form hose at the jet nozzle of the system and improving the seal's performance of the check valve. What should be pay attention to in selecting the rupture disk is its material, explosive pressure and applied location. The under-liquid form extinguishing technology has a wide application perspective on the fixed roof tanks which store gasoline, diesel oil, kerosine, lube oil and crude oil.