A new method for producing tunable ns duration pulse trains was presented in refrenoe[3]. The oscillating-disoontinuous amplifying model for investigating tunable ns pulse trains is set up in this paper. It consists of three groups of the rate equations, which represent three stages respectively』that is the oscillating,"the resting and amplifying stage in the process of pulse trains formation. Based on this model, the gain of medium, amplitude and spectral behavior of the pulse trains are calculated,the process of forming and developing pulse trains is investigated. The results indicate: (1) The gain coefficient fluctuates across the threshold in a period of the pulse trainSjit appears as tlie gain sainration. (2) The degree of pulse modulation and the looking efficiency of frequency decrease with the time in some degree, while within 0.4μs the degree of the pulse modulation is better than 98%, the looking efficiency of frequency is better than 95%. The pulse amplitude of narrow band operating approaches to that of broadband operating, it means that, the energy loss caused by narrowing spectrum is only a few. The theoretical results are in agreement with experiments basically.