We propose a full-band model to quantitatively calculate terrestrial atmospheric scattering in stray light testing based on the Monte Carlo algorithm. Measurements are conducted using two classifications of air cleanliness at off-axis angles from 35° to 90°. Corresponding simulations of Mie scattering and Rayleigh scattering are used for a comparison with the measured values. The relative root mean square deviation of the simulation from the measurement result is 3.72% and 24.1% for Mie scattering and Rayleigh scattering, respectively. This exhibits excellent agreement between the measured and predicted values for a 26° full-angle baffle when illuminated by a 550 mm diameter collimated beam.