中文摘要：

在股权分散的上市公司中，存在着全体股东与经营者之间的单层委托代理关系，而在股权集中的上市公司中，存在着大股东与经营者、中小股东与大股东之间的双重委托代理关系。将会计政策变更作为一个可观测变量，针对股权分散和股权集中的两类上市公司，分别建立单委托、双重委托代理模型，分析降低代理成本、提高公司价值的途径。分析结果表明，在两类上市公司中，将会计政策变更作为一种信号传递机制写入激励合同，可以降低代理成本，提高公司价值。

英文摘要：

In Chinese listed companies, with decentralized ownership, there is single principal-agent relationship between all stakeholders and managers. But Chinese listed companies with centralized ownership have double principal-agent relationships between large shareholders and managers and between small share- holders and large shareholders. This paper builds single and double principal -agent theory models respectively for two different listed companies and study measures to reduce agent cost by using accounting changes as an observable variable. The result shows that accounting changes, which is a signaling mechanism, can reduce agency cost and improve company value.