Extraction and purification method for available boron in water sample and testing method of composition of boron isotope

The invention provides an extraction and purification method for available boron in a water sample and a testing method of a composition of a boron isotope. The extraction and purification method comprises the steps: firstly, regulating the pH value of the water sample to 6-7, and placing the water sample after the adjustment of the pH value into a cap of a centrifugal tube; placing a sodium hydroxide solution at the bottom of the centrifugal tube; heating a neutral water sample in the cap to obtain a sodium borate solution at the bottom of the centrifugal tube; and then, sequentially carryingout boron specific resin adsorption treatment, hydrochloric acid leaching, leacheate evaporation treatment and anionic and cationic mixed resin adsorption treatment on the sodium borate solution to realize the extraction and purification of available boron in the water sample. The boron isotope in a to-be-tested solution of available boron obtained by extraction and purification is tested by using a positive thermal ioni