Based on the collisionless, Maxewellian Thomson theory, a program is developed for carefully analyzing the influence of experimental set-up and plasma gradients and electron drifts on Thomson scattering spectrum. Calculations show that spectra resolution, collection angle, drift velocity and plasma gradient only have minor effects on diagnostics, so we can conveniently ignore these in fitting the spectrum. An important result is that signal-to-noise ratio can be enhanced by increasing the collection angle of the experimental system. The uncertainty of the scattering parameter can strongly affect the inferred plasma parameters when it is less than 3 This problem could be solved by choosing a smaller scattering angle or by accurately measuring the plasmas density.