In order to measure the stray light,which is one of the important influence factors in spectral accuracy,the definition, source and harmfulness of stray light are introduced. Cut-off filter method, spectrum method, spectral stray light factors method and so on are presented to discuss the advantages and disadvantages of normally used stray light measurement methods. Then, the feasibility and superiority of the stray light factor used to describe the stray light characteristics of the spectral instruments, is investigated. At last, the system components, measurement procedure and measurement results of the stray light measurement system, using narrow-band filter, are introduced, and the uncertainty of the results is analysed. Results show that the stray light factor indicates the stray light characteristics of the spectral instruments effectively without relationship to the light source, detector and other measurement conditions. The relative uncertainty of the measurement is 0.646% when the fiducial probability is about 95%, which can meet the needs of the stray light measurement.