The invention relates to the technical field of detection, in particular to a detecting method of the 
nitrogen content of a cotton 
plant. The detecting method comprises the following steps: step one,sampling: taking fresh cotton 
plant leaves; step two, preparation of a 
grinding device: preparing the 
grinding device; step three, sample treatment: 
processing the sample obtained in step one by adopting the 
grinding device, and forming a smashed sample; step four, 
digestion: adding the smashed sample obtained in step three in concentrated 
sulfuric acid, and uniformly mixing; then heating the 
mixed solution, and heating until the solution is a uniform brownish black color after generating white 
smoke; stopping heating, and cooling; adding 
hydrogen peroxide after cooling, heating until boiling,and digesting for 6 to 12 minutes; cooling again, and adding 
hydrogen peroxide; digesting until the solution is colorless or clear, and heating for 10 to 12 minutes again; finally taking and coolingsample liquid; step five, filtering by adopting 
filter paper or standing and clarifying the sample liquid cooled in the step four, taking 
clear liquid, and determining the 
nitrogen content. Accordingto the scheme disclosed by the invention, the problem that 
decomposition and extraction of 
plant leaves in the prior art are incomplete is solved.