The invention discloses a testing system of nitrogen content of nitrocotton and a detecting method of nitrogen content. The system comprises a front cover, a sealing handle, a steam tube, a saponifying tube, a water adding barrel, a boric acid adding barrel, a standard-hydrochloric-acid adding barrel, a tray, an operating disc, a condensate tube, a boric-acid absorption liquid adding tube, a standard hydrochloric acid solution adding tube, an absorption bottle, a power switch and an optical sensor. The system is mainly used for testing the nitrogen content of the nitrocotton. The detecting method of the nitrogen content by adopting the automatic testing system of the nitrogen content of the nitrocotton comprises main steps as follows: sampling, saponification digestion, reduction, distillation, absorption and titration as well as data processing and calculation. The system and the method have the characteristics of higher accuracy and precision of tested data; the test is convenient to operate and easy to implement, and the automatic degree is high; the cost of consumables in the test is lower, and the testing system and the detection method are suitable for being applied to and popularized in nitrocotton production and application enterprises.