The grassland ANPP were calculated with CASA model and tested by field data in the Three River Headwater region from 2000 to 2014. The dynamics of grassland ANPP and its relationships with climatic factors (temperature, precipitation) and carrying capacity were analyzed. The results indicated that (1) average annual ANPP from 2000 to 2014 was 27.53 g C/m~2 in the Three River Headwater region, in where about 75.81% and 24.19% areas showed increasing and decreasing trends. (2) ANPP showed a significantly positive relationship with increased mean annual temperature if take the Three River Headwater region as a whole, which showed a parabolic pattern with temperature under different precipitation. (3) the relationship of ANPP with precipitation depend on the temperature, which showed a positive pattern in where annual temperature above -2℃,but with no changes in where annual temperature below -6℃,and in the areas between -6 ~ -2 ℃ showed a parabolic pattern. (4) the response of carrying capacity to ANPP of grassland in the Three River Headwater region had a time lag, ANPP did not show obviously correlation with current year's carrying capacity, while presented a significantly correlation with that of next year.