A method for globally modeling and analyzing grating spectrometers is put forward in the present paper.Different from existing methods which are confined to parts of a spectrometer,the method takes a grating spectrometer as four functional parts,namely imaging optics,detector,reconstruction and display.Effects of each part on spectrum are considered and a global model of the spectrometer is developed,accompanied with its transfer function.With the help of the model and the transfer function,laws of each part affecting the holistic performance are summed up.It is suggested that high quality spectrogram needs enhancing baseband response and reducing spurious response,and reconstruction is an effective way.