Group decision that multi-interest agents take part in is an important method, which can promote the democratization and scientific process in the water resources allocation. And the weight of multi-interest agents play an important part in the efficiency of group decision. This paper proposes a method of modifying weight based on similarity measures of intuitionistic fuzzy matrix after multi-interest agents' negotiation about weight. The method can show both subjective and objective degrees of decision making, and make the final decision more reliable. The case conducted that the method is feasible.