Hyperspectral imager has more spectral channels and higher spectral resolution compared to normal spectral instrument. Stray light is one of the important reasons affecting the spectral accuracy of the hyperspectral imager, but the current stray light measurements can’t meet the need. Definition, sources and harmfulness of stray light are described in the present paper. The feasibility and superiority of the stray light factor di,j, used to describe the stray light characteristics of the spectral instruments, is investigated thoroughly, where the definition, physical significance and value of engineering application of stray light interference factor fi(λ) and stray light disturbance factor Fi(λ) are given. Finally, the system components, measurement procedure and measurement results of the stray light measurement system, using narrow-band filter, are introduced. Results show that the stray light factor di,j meets the need of the stray light measurement because it can indicate the stray light characteristics of the spectral instruments without relationship to the light source, filter, detector and other measurement conditions, and the measurement efficiency is increased at least one-fold than the spectral stray light factors method.