In view of the uncertainty of water supply and demand in a water resources system and its dynamics with the passage of time, based on the data of water supply, water consumption and social economy in Heilongjiang Province over the years, the water resources parameters and economic parameters in the planning years were determined. The uncertainty in the system was reflected by using interval parameters, and the dynamics in water resources allocation was reflected by using multi-stage random planning. An interval multi-stage random planning model with multiple water sources and multi-purpose users was then established, based on which the water resources allocation in Heilongjiang Province was studied. The results show that the domestic, industrial and ecological water consumption in Heilongjiang can be met in the planning years, and the agricultural water consumption is basically met in wet years, with the water shortage of 1.532 billion to 4.94 billion m³ in dry years. Compared with the current years, the agricultural water shortage has been alleviated to some extent. In terms of water supply, the proportion of surface water supply steadily increases, which can better protect underground water sources.