A comprehensive analysis has been made for straightness errors of feeding guide of grating ruling machine, and some mathematical expressions have been deduced among the straightness errors, the diffraction wave- front and the intensity of stray light, and the distance from the measurement axis of interferometer to the beginning of lines has been defined as position arm. Some following conclusions have been made. Position arm can magnify the straightness errors of dividing guide. The diffraction wave-front and the intensity of straight light are mainly affected by the position arm and the straightness errors of feeding guide. If the position arm is 50 mm, the horizontal straightness errors must be less than 0.15″. However, if the position arm is 10 mm, the horizontal straightness errors must be less than 0.74″ . Compared with the straightness in the vertical direction, the requirement of the grating- ruling machine for the horizontal straightness of dividing guide is higher. The value of theoretical calculation coincides with the experimental results.