This paper aims to build a quiet Sun EUV reference spectrum from the SOHO - SUMER database. We make use of 750 quiet Sun spectra spread over a large range of solar activity. They are carefully calibrated both in terms of frequency and intensity. The data do not show any noticeable variations versus the solar activity. This allows us to build a quiet Sun reference spectrum from data scattered over any solar activity level. We consider separately the continuum, the thick-line spectra, and the thin-line spectra. The former is computed from an average of the H Lyman and C I continua. The thick lines are directly assembled from the SUMER files. To retrieve a broad set of thin lines, we use the SUMER measurements to compute a mean Differential Emission Measure, which is then inverted. This results in a reference spectrum for the Quiet Sun, which is compared to previous studies.