Welding flux, such as fused and agglomerated flux, are composed of various inorganic and organic minerals such as silicates, carbonates, oxides, fluorides, iron alloys and metal powder. They are prepared by smelting, sintering, binding or mixing. Welding flux would play an important role in slagging, deoxidation, gas making, arc stabilization and alloying in the fusion welding of steel and nickel-based alloys. They have been widely used in the engineering fields, including shipbuilding, ocean engineering, energy, metallurgy, chemical, and machinery. Phosphorus is a trace and harmful impurity element in iron, steel and nickel-based alloy materials, which could reduce the mechanical properties of materials by causing embrittlement and micro-cracks in deposited metals and welded seams. The accurate and rapid determination of phosphorus in welding flux is significant to performance evaluation and quality control. Working conditions for determining phosphorus in welding flux by inductively coupled plasma atomic emission spectrometry with ultrasonic-assisted hydrochloric acid were optimized, such as radio frequency power, gas flows of atomization, auxiliary and plasma, and analysis line. The optimized operating parameters were obtained below: The radio frequency power was set to 1.2 kW, and the gas flows of atomization, auxiliary and plasma were set to 0.75, 1.0 and 12 L·min^-1, respectively. P 213.617 nm was selected as the analysis line. The sample suspension was prepared with a 0.5 g welding flux sample in 20 mL hydrochloric acid with ultrasonic-assisted extraction for 30 min and was heated to boiling with 2 mL nitric acid. After cooling, the volume of suspension was adjusted to 100 mL by pure water. Then the suspension was dry filtered, and the filtrate was used for measurement. A series of solutions for developing a calibration curve was prepared with 20 mL hydrochloric acid and 2 mL nitric acid as the matrix. The linear correlation coefficient of the calibration curve was 0.999 9, and the quantification limit was 0.0012%. The actual sample and certified reference materials of welding fluxes with phosphorus between 0.003% to 0.03% were determined, and the results were consistent with the certified or obtained the standard. The relative standard deviations (n=6~10) of determination results were less than 8%, and the recoveries were between 93% to 110%. The proposed method has the advantages of accuracy, fast and convenience. It is suitable for the rapid inspection of phosphorus in welding fluxes.