This paper pointed some errors in physical concepts and formula deduction in the reference [1]. Correcting those errors and considering the real experimental geometry and the scales of laser plasma sources, we have performed a systematic simulation for the characteristics of the grazing incidence grating spectrograph with toroidal mirror X-ray relay optics which has been used in X-ray laser experiments for gain measurements. It is shown that the nonlinear effects of the spectrograph in optical energy transportation caused by defocusing is negligible, and so is the related defect in spatial resolution of the spectrograph.