Relationship between secondary electron yield (SEY) and electron incident angle has been measured for a polyimide sample. SEY as a function of incident angle at different incident electron energy is measured by use of a system with a single pulsed electron beam and a special surface charge neutralization technology based on the negatively biased collector. Measured results show that the SEY may deviate from the traditional law of monotonic increase with the incident angle when the angle is higher than a certain critical value. This deviation is even more obvious at lower incident electron energy. The critical incident angle decreases with decreasing incident energy. A theoretical analysis on the deviation is given in a simplified electron elastic scattering process. The distribution of the scattering region has an important effect on the relation of SEY versus incident angles. A sector region is introduced to describe the electron scattering region. Due to the limit of sample surface, the electron scattering region will decrease if the angle between the incident direction and the sample surface is smaller than half of the central angle of the sector. Corresponding SEY might no longer increase. Based on the Rutherford’s elastic scattering formula, a formula for the critical incident angle is derived as a function of incident electron energy, which is also confirmed by our measurement results. Finally, a revised SEY computation formula is developed which can give more accurate results at high incident electron angle.