In the process of fair trial, the district courts in Iran have legislative gaps in their structure and performance and this have provided a deviation from the principles and regulations of criminal justice. The purpose of this study is to recognize, evaluate and explain the obstacles and challenges of fair trial in the preliminary investigation stage in the district court and its solutions. The data analysis method is descriptive and analytical. The findings show that due to the fact that there is no prosecutor's office in the district courts and the prosecution and preliminary investigations and trial by a single judge are carried out in one stage, the judicial process and supervision are distorted and the guarantees of the fair trial pattern are difficult and the due process is distorted, especially during the preliminary investigation phase. In order to overcome these challenges, it is necessary to reform the structure of District Court based on guarantees of fair trial model Objectively, sensible and accessible to individuals, as well as establishing the characteristics of the mixed litigation system and the principle of separating prosecution officials from independent investigative authorities with judicial supervision.