Based on labor theory of value and marginal utilitytheory of value, this paper proposed the marginal labor value determinism of value and established the theory and method for evaluating the economic value of multi-purpose water.The economic value of water should depend on the minimum value of water benefits for various use or the maximum value of water cost at the balance of supply and demand. Based on this theory, the economic value of raw water in Beijing was assessed. The results show that the economic value of water in Beijing is defined by agricultural water, and the estimated value of agricultural water benefit in Beijing is closer to the value of water. Combined with the South-to-North water price, which can be regarded as the marginal cost of water supply in Beijing, the economic value of raw water in Beijing in 2020 should be between 2.33 and 4.82 RMB Yuan per cubic meter.