The experiment's datas processing method of measuring grating constant by the maximum angle of deflection is studied. The reason for the large error of grating constant measuring value is analyzed, and the scheme of diminishing the errors is brought forward. The least square method is introduced in processing the experimental data, which seems to be reasonable and accurate here. The corresponding data processing procedure is compiled in MATLAB software. The curve equations satisfied the scattering data are fitted by the maximum angle of deflection. The position of disappearing point is solved by simultaneous equations, which can solve availably the difficult question to ascertain the position of disappearing point accurately.