中文摘要：

大型项目通常由几个分阶段的任务组成，比如修建公路时，需要前期的建设和后期的运营维护。委托人可以将任务捆绑在一起招标，也可以分开招标。本文首先刻画了在捆绑和非捆绑两种情况下的最优拍卖机制，证明了两种情况下最优的拍卖机制都可以通过简单的线性合约和参与费用来执行；并且如果任务之间呈现出足够大的正外部性，那么委托人会偏好捆绑拍卖，反之，则会偏好非捆绑拍卖；最后证明了当竞拍者的进入是内生时，捆绑会削弱竞争。

英文摘要：

This paper studies the optimal auction design in PPP. When a project composes several tasks, the principal can bundle the two tasks together to hold an auction or auction them separately. We characterize the optimal auction under the two forms. We prove that under both cases the optimal auction can be implemented by a linear contract with participation fee. If there are enough positive externalities between the two tasks, the principal prefers the bundling auction and vice versa. We also prove that if the entry of the bidders is endogenous, bundling auction can reduce competition which will hurt the principal.