Focusing on the residual amount of liquid ammonia in modified cotton yarn, this paper presents a new method of drying the liquid ammonia by microwave. An experimental system is designed to find whether this new method is correct and to analyse the effect of microwave drying on the dyeing rate of cutton yarn. Firstly, the microwave drying experiment is carried out to prove that this new drying method is better than the traditional steam drying way. Then, another contrast experiment on the dyeing rate confirms the good effect of microwave drying. So it is concluded that microwave drying is an efficient and energy-saving way to remove the remaining liquid ammonia. Meanwhile, the experimental results provide the relevant data to make guiding sense to engineering application.