The invention provides a core spun yarn and application thereof. The core spun yarn comprises a core layer and an outer layer wrapping the core layer, the core layer is a mixture comprising vanadium nitrogen alloy, ferro-silicon nitride, iron alloy and aluminum pills, and the iron alloy is titanium iron alloy and / or niobium iron alloy. The invention provides vanadium nitrogen alloyed molten steel and a preparation method thereof, and the method includes feeding of the core spun yarn into to-be-vanadium and nitrogen alloyed molten steel for vanadium nitrogen alloying. The invention provides vanadium nitrogen microalloyed steel, and the vanadium nitrogen microalloyed steel is casted by the vanadium nitrogen alloyed molten steel. The core spun yarn is used for realizing molten steel vanadium and nitrogen alloying, nitrogen content can be effectively increased without affecting on the vanadium content according to the need, vanadium containing steel with higher nitrogen content can be produced in the need of the molten steel, and the recovery rate of nitrogen is high and stable.