This article deals with the resources of the stray light in holographic concave grating monochromators. A new plan of measuring the stray light is suggested. The linear calibration in large-order-scale is solved by using light intensity recursion, real environment calibration and photon counting technique. The measuring limit of order 10-12 is obtained under the condition of the sampling time of the photon counter in one second. This article introduces a new designing idea for eliminating the error caused by the fluctuation of light source intensity and for improving the precision and accuracy of experiment. Finally, the measurement results of the stray light in the monochromatic system of the Raman spectroscopic instrument designed and produced by our institute are given.