In this paper, the problem of multi-attribute procurement auction of divisible goods is investigated, and an optimal mechanism of multi-attribute auction, which enables negotiation on several attributes in addition to the price of a divisible good, is designed. First, several important assumptions of multi-attribute procurement auctions of divisible goods are given, and the buyer's utility function and the supplier's utility function are defined. Then, a set of sufficient conditions for feasible multi-attribute auction mechanism are given. Based on these conditions, an optimal model of multi-attribute procurement auction whose goal is to maximize the buyer's expected utility, is established. By solving this model, the optimal allocation strategies are obtained. Third, the properties of the optimal multi-attribute auction are discussed. Finally, a multi-attribute auction example about the steam coal procurement is given to show how to implement the optimal multi-attribute auction mechanism.