Abstract: Based on conservation of mass and momentum, a fluid dynamics model of debris motion from a near-space nuclear detonation is established. Many influence factors are considered, such as energy dissipation, air density as a function of height, gravity, increasing air temperature caused by X-ray deposition and radiation cooling. A computation program is developed. The expansion and upward motion of debris in TEAK and ORANGE tests are calculated. Evolution of typical parameters such as central height, maximum horizontal radius, ascending velocity, and shape of debris are given. They are in accordance with experimental data. The model provides delayed radiation source parameters for ionospheric effect and artificial radiation band effect.