According to data of soil water, grass biomass output and meteorological factors in recent 20 years, and by methods of contrasting analysis and water quantity balance, the empirical equations of water requirement and evapotranspiration rate changing with time in growth season of the meadow, the typical and the desert steppes were formulated in the paper. The water requirements of the meadow, the typical and the desert steppes were respectively 390 mm, 360 mm and 180 mm during the period from April to September, their corresponding evaportranspiration rates (Kc) were 0.62, 0.58 and 0.30. Based on the principle of SPAC, the calculating method of steppe drought index（K）was provided, which was combined with the reduction rate of grass yield (I) the drought grading index system was further put forward. Slight drought: 0.8＞K≥0.6，25%＜I≤45%; moderate drought: 0.6＞K≥0.4，45%＜I≤65%; Severe drought: K＜0.4, I＞65%. The consistency of the index system reached 75.3% in three years test, and the drought description of this system was generally identical with that of the animal husbandry meteorological report.