Defining abnormal transmittance as the case where the magnitude of the shortwave flux transmittance is greater than 1.0, the authors used surface solar irradiance and all-sky images obtained at the Yangbajing site in Tibet to analyze the reasons for the occurrence of abnormal shortwave flux transmittance. Based on the Three-Dimensional Radiation Code (I3RC) Monte Carlo community model of three-dimensional radiative transfer, the authors also performed simulations at a nonabsorbing wavelength and an absorbing wavelength through a stratocumulus and a cumulus field. The results showed the detection of abnormal transmittance on more than half the days, and the maximum transmittance was 1.34. The probability of the occurrence of abnormal transmittance appeared to be largest in summer, and on a daily basis was mainly at about noon local time. Abnormal transmittance mainly appeared when clear sky and clouds co-existed, especially at the edges of broken clouds and nearby regions with clear-sky conditions. The flux transmittance decreased as the solar zenith angle increased.