A model is built from the infrared remote sensing radiative transfer theory and the two stream approximation for passive infrared detection of biological aerosol,which theoretically illustrates the effect of scattering of radiances from all sides incident on the aerosol. The laboratory principal experiments are conducted on a microorganism spores aerosol for studying the infrared spectral properties of biological aerosol. The results of preliminary laboratory experiments in bioaerosol cell are presented,and the experimental phenomena are explained with radiative transfer model. It is shown that the detection model are effective and reasonable. Combining the detection model and the laboratory experimental results can give a reference for field passive standoff detection experiments of biological aerosol.