Abstract: The quality and wearability of milk protein fabric would be influenced by its protein component that will be hydrolyzed and run off by humidity, heat, acid，alkali, and oxidizer during dyeing and finishing. For this reason, this paper tried to determine the change of the protein amount in milk protein fiber during the dyeing and finishing process by detecting the change of nitrogen content via the optimized Kjeldahl Nitrogen Analysis. The research revealed that the optimized Kjeldahl Nitrogen Analysis had the advantages of short digestion time, high precision, and easy operation. The change rule of protein content of milk protein fiber during wet and thermal treatment, bleaching, acid or alkaline treatment and blank dyeing was revealed by this test method. This research work may give a guidance for preparing the dyeing and finishing process of milk protein fiber.