The spectral width of cloud droplet spectra is crucial to the parameterization of the cloud optical depth, the assessment of the indirect aerosol effect and the formation of precipitation in numerical simulation. Based on the aircraft observations of cloud microphysics in stratocumulus on July 19, 2008 during the Physics of Stratocumulus Top (POST) field campaign, the vertical distribution of microphysical properties and cloud droplet spectra are analyzed, as well as the cloud microphysical processes. The results show that the spectral width of cloud droplet spectra is basically larger near cloud base which is caused by aerosol nucleation. The spectral width decreases as height increases in the middle layer of the cloud, which is caused by condensation growth. The increase of the spectral width near the cloud top is caused by the entrainment-mixing processes. The increase of vertical velocity in the adiabatic cloud increases the cloud droplet number concentration by promoting the activation of cloud condensation nuclei (CCN), and increases the cloud droplet size and leads to the decrease of the spectral width by promoting the condensation growth, and ultimately leads to the negative correlation between the spectral width and the cloud droplet number concentration as well as the cloud droplet size. The decrease of vertical velocity caused by entrainment-mixing processes in cloud holes decreases the cloud droplet number concentration and the cloud droplet size and increases the spectral width by promoting the evaporation of cloud droplets, which will be enhanced by the decrease of adiabatic fraction. Based on these observations, it is recommended that the parameterization of the spectral width of cloud droplet spectra takes into account vertical velocity, cloud droplet number concentration, cloud droplet size, adiabatic fraction, etc.