Neutron capture γ-rays from nitrogen is an important part of initial nuclear radiation. To accurately calculate the early nuclear radiation, the Monte Carlo computing model for the neutron and secondary γ-rays transport in the atmosphere is established. Some variance reduction techniques, such as geometric splitting algorithm and time splitting algorithm are used in the Monte Carlo method to improve computing efficiency. The neutron and secondary γ-rays information on the spherical surface at different distances are calculated. The energy release rates of capture γ-rays from nitrogen at different positions and at different times are given. The regularity of the energy release rate of capture γ-rays from nitrogen is studied, and the influence of neutron energy on capture γ-rays from nitrogen is analyzed. The results show that the energy release rate of capture γ from nitrogen increases with the increase of distance from the source at first, reaches the peak at about 500 m from the source, and then decreases exponentially with the increase of distance. The energy release rate of capture γ-rays from nitrogen obeys exponential decay law in time, and the decay time is about 0.1 s. By introducing the parameter of a representing the radiation intensity and the characteristic attenuation time parameter of τ, a fast calculation formula for the energy release rate of capture γ-rays from nitrogen at different distances and times is obtained by fitting formulas. Results show that the intensity radiation, the decay time scale of capture γ-rays from nitrogen and their spatial distribution are closely related to neutron energy.