As one of important technical specifications of diffraction gratings, stray light directly affects Signal Noise Ratios(SNRs) of spectrometers; especially in spectrum analysis at UV spectral bands.In order to search the stray light from applications of ruled gratings in the spectrometers, this paper analyzes forming causes of the stray light in theory based on a scalar diffraction equation.The numerical simulation indicates that the random errors in the spacing of grating grooves and the random errors in the groove depths for UV ruled gratings are the main sources of the stray light, moreover, the small-scale random surface roughness affects the stray light slightly.Otherwise, it presents a concept of a relative width for the exit slit and establishes the relation between the height and relative width of exit slit and the intensity of the stray light,which provides a theoretical reference for reducing the stray light in the process of fabricating gratings essentially and repressing the stray light in practical applications.Finally, in order to compare theory results with experimental results measured by a filter method, an equation to solve the total stray light is presented to measure the stray light of four wavelengths by a UV spectrophotometer.It proves that the maximal relative error between theory results and experimental results is about 13%, when random errors in the groove periods, random errors in the groove depths and random surface roughness are 0.8 nm, 0.5 nm and 1.2 nm respectively.