The main propose of this paper is to discuss the possibility of a space-based early warning technology for missiles in boost phase based on the near-infrared fine spectrum of potassium atoms in the exhaust plume. Emission transfer link from the exhaust plume to the detector is established in combination with the observation model of satellite and target on the ground. Line-by-line integral method is used to calculate the characteristic spectrum of potassium atoms. The result shows the potassium line have high spectral emissivity and narrow bandwidth. The analyses on the atmospheric transmission and background radiation indicate that the atmospheric transmission of the 769.896 nm potassium line is higher than that of the 766.490 nm potassium line which lies on top of an O-2 line, and the irradiance of the 769.896 nm line is stronger than that of background and the 766.490 nm line. Considering atmospheric transmission and background radiation, it is suitable to choose the 769.896 nm line to detect the exhaust plume of the missile. According to the characteristic of potassium atoms emission line with narrow bandwidth, a 1.2 nm wide filter centered on 770nm is used to extract target signal. The maximum detection range and other indexes are evaluated. The simulation results show that ultra-narrow band filter can achieve a large degree of background suppression, and the system performance indexes meet the detection requirements. Therefore, it is feasible that missile detection can be realized by using near-infrared fine spectrum of potassium atoms.