Water resources shortage has become one of the important factors restricting the economic development and the social stability of Beijing City, so it is a fatal time to coordinate urban greening and water resource. Based on the current situation of urban greening and water utilization in Beijing City, the authors calculated the ecological water demand of urban greening by scale extending computing methods, and by effective utilization of rainfall combined with survey data to calculate the theoretical and actual water supply of Beijng's urban greening in 2012. On this basis, the three sources of Beijing's water supply for urban greening, which including surface water, ground water and reclaimed water, were evaluated by the gray Markov chain model and entropy evaluation method, thus obtained the predicted value of water actual supply in 2014. The results indicated that the theoretical water supply was 5.07×10⁸ m³ and the actual amount of water was 6.86×10⁸ m³ in 2012, possible range for the actual amount of water is between 7.29×10⁸ m³ and 7.93×10⁸ m³ in 2014. Accordingly, there have risks of Beijing's greening water supply and demand, so the authors provides some advice about water-saving measures and related research references to ensure Beijing's urban water supply security and sustainable utilization of water resource.