中文摘要：

在正常的聘任关系下，学习效应有助于审计技能的提高。因此，随着签字审计师任期的延长，审计质量会逐渐提高，但在异常聘任关系下，由于签字审计师与客户存在较为密切的私人关系，审计师的独立性可能因此受损，学习效应对审计质量的积极影响也会受到削弱，因此，随着签字审计师任期的延长，审计质量可能会变差。本文的经验证据发现，尽管总体上同美国审计市场一样，审计师任期的延长有助于审计质量的提高，但是在细分审计师与客户之间的关系后，本文的经验证据支持上述理论。

英文摘要：

In normal audit-engagement, audit quality may get better with the extension of tenure of signing auditor due to the improvement of audit technology derived from study effect. But, in abnormal audit engagement, since auditor＇s independence may be impaired and further study effect of a positive impact on audit quality will be weakened due to the private relationship between signing auditor and client, the audit quality may become worse with the extension of tenure of signing auditor. Our results address that, although, as a whole, the extension of audit tenure helps to improve the audit quality like American audit market, our evidence supports the above anticipation when we divide the relationship between auditor and client into two categories.