The influence of pump beam quality on Raman generator at low pressure is discussed. The experimental results of energy threshold, output energy and beam divergence of the first Stokes are present. The quality of pump and the first Stokes beam is analyzed using the M 2 concept. The energy threshold of the first Stokes is calculated according to an approximate theory including the effect of focused pump beam and beam quality. The results are compared with experimants and theory in related references.