Ammonia nitrogen is a common pollutant in water,and it is an important indicator of various water quality detection.Excessive ammonia nitrogen content in water will affect the growth of aquatic plants and animals,which will lead to water environment d eterioration and water environment pollution.Therefore,it is very important to strengthen t he monitoring of ammonia nitrogen in water.In this paper,based on the UV spectrum detection technology, the UV spectrum detection system of ammonia nitrogen concentration in water was built. Through the detection system,ammonia nitrogen in liquid phase is converted into gas phase ammonia,and the ultraviolet absorbance of ammonia gas in the wavelength range o f 190-235nm is obtained,and the absorbance signal is analyzed by Fourier fi ltering method and Fourier transform.The results show that after Fourier filtering,the absorbance of ammonia has obvious periodic peaks in the wavelength range of 190－235nm,and the perio d is about 4nm.With the increase of ammonia concentration,the absorbance peak a lso increases.It was found that there was a good linear relationship between the absorbance and c oncentration of ammonia by least square method,and the correlation coefficient was 0.99814.After Fourier transform,the absorbance signal has characteristic frequency in frequen cy domain. Through fitting,it is found that there is also a linear relationship between am plitude intensity and ammonia nitrogen concentration,and the correlation coefficient is 0.98686.The experimental results show that the concentration of ammonia nitrogen can be obta ined according to the absorbance of ammonia,which provides a new detection method fo r the concentration of ammonia nitrogen in water.