Aiming to the optical transmittance measurement using integrating sphere as optical concentrator,there exists a non-normal situation that the measured value of transmittance exceeds 100%.A model of the relationship between the output illuminance of integrating sphere and the incident beam geometry was developed to aid in a simulation experiment.For a 300 mm diameter and 1% core-opening ratio integrating sphere,the relative error of output illuminance due to changes in the incident beam geometry is 3.4% and the error of transmittance measurement due to changes in the incident beam geometry is 3.4%,which coincides with actual measurement result.Simulation experimental result indicates that the random placement and distribution of spot is the main error source which leads to the measured value of transmittance over 100%.The error source is concealed and inevitable,so it is of key theoretical and engineering value to take account of it for improving the accuracy of luminosity measurement using integrating sphere.