Most people have no idea of paying in debt and there is no specific definition in China laws. By the increasing development of economy, various method of payments have been emerging. Paying in the dept has prevailed. It is obvious that debtor can not pay the debt in cash thereby seeking substitutes to pay the debt. Paying in dept protects parties’ benefits effectively, which is one of main reasons why judges can not deny it. However, it creates many challenges when judges confront cases. First of all, there is no written and clear regulation. Therefore, people find it difficult to decide whether payment in debt or not. Besides, there are many methods of paying in dept in practices, such as discharge by substitutes, Fluidity contract, modification of debt, New debt repayment、 alienation guarantee etc. We need to clarify features of paying in dept and components of paying in dept before discussing the pay in dept. By researching the definition, catalogs and features we can have better understand on paying in dept. Seeking valuable theories and regulations from previous researches, in order to make clear definition of contract of paying in dept and its effects. It is worth noting that in comparison with other laws, we can demonstrate the irreplaceable of paying in dept, which helps people to set up explicit laws and regulations.