It is important for the echelle spectrograph to acquire the exact positions of the spots in the two-dimension spectra image, which directly influence the precision of spectral analysis. With the target of high resolution, which is 15 000 in the echelle spectrograph being discussed, the acquired positional error must be less than 0.03 mm-which equals 2 pixels. According to the characters of the two-dimensional spectra, a position-acquiring method for the signal spots based on the centroid computing algorithm was put forward. Applying this method, the signal spots were distinguished from the noise spots and their positions can be acquired accurately and swiftly, with the positional error less than 2 pixels and wavelength error 0.02 nm, which satisfy the requirements of the echelle spectrograph.