To study t he impact of grass land degradation in Yellow river source,a control experiment and a sensitive one making grassland degradation in Yellow river source were designed and inte-grated for whole mont h of July , 2003 using t he mesoscal model MM5 and t he NCEP reanalysed Data. It was found that the grassland degradation induced obviously t he 2 m height air temperature and ground surface temperature ascend ; the air humidity and soil moisture , decreasing in different extent ; the runoff , reducing ; t he sensible heat flux increasing , t he latent heat flux and the efficient flux (sensible heat flux plus latent heat flux) , decreasing. The influence of grassland degradation on climate environment was stronger than that of the grassland agricultural-pastoralization.