Based on the experiment data of field simulative rainfall,the seasonal change of coverageof two years old artificial grassland of Astragulus adsurgens and Melilotus albus and their benefits ofsoil and water conservation were analysed and discussed in this paper.The results are showed as fol-low the seasonal change of coverage was power distribution;the correlation are exponent functionbetween coverage and the time of runoff beginning,or runoff amount,or sediment yield seperately,the peak value of coverage of artificial grassland appeared at the same time as the peak value of year-ly rainfall dstribution appeared.Meanwhile,bacause of high coverage,the benefit of soil and waterconservation was very good.