Based on surface observational data obtained at Beiluhe site (degraded alpine meadow) and Maqu site (alpine grassland) over Qinghai-Xizang Plateau in 2014, a combinatorial stratification method was used to compare and analyze direct and indirect effects of soil water content, vapor pressure deficit and net radiation on surface energy partitioning. A path analysis method was used to study the key climatic factors that influence the surface energy partitioning on two types of underlying surface. The results show that the response of evaporative fraction(the ratio of latent heat flux to the available energy of the land surface) to soil water content stays in the soil moisture-limited regime at Beiluhe site and energy-limited regime at Maqu site. Evaporative fraction grows fast with increases in soil water content at high levels of vapor pressure deficit and net radiation has little impact on the variation of evaporative fraction at Beiluhe site. The variation of evaporative fraction with changes in soil water content is not influenced by vapor pressure deficit and net radiation at Maqu site. Evaporative fraction first decreases and then tends to be stable as vapor pressure deficit increases, and the sensitivity of evaporative fraction to vapor pressure deficit decreases when soil water content becomes larger while it is independent of net radiation at Beiluhe site. Evaporative fraction increases slightly as vapor pressure deficit increases at first, and then becomes stable and is no longer influenced by soil water content and net radiation at Maqu site. At both sites, evaporative fraction tends to become stable as net radiation increases and the stable value is determined by soil water content at Beiluhe site and vapor pressure deficit at Maqu site. Results from path analysis indicate that precipitation is the dominant factor that controls evaporative fraction at Beiluhe site and air temperature is the dominant factor for evaporative fraction at Maqu site.