The Simjurids family played a major role in the decline and weakening of the Samanids. For the Simjurids, who were Turkish slaves of the Samanid court, the position of the sepehate and the feudal lordship of Qohestan was a tool that enabled them to provide the grounds for the deterioration and weakening of the Samanids. From the beginning of the Samanid's rise to power, the Simjurids, in addition to the position of sepehate, were able to gain sovereignty over Qohestan and maintain it until the end of the Samanid rule and even the beginning of the Ghaznavid period. Abul Hasan Simjuri was the general of Khorasan for nearly thirty years during the Samani regime. His son Abu Ali increased his autonomy and weakened the Samanids. At the end of the Samanid government, weakness dominated all the Samanid rulers and authorities, and Khorasan was in chaos. The Simjurids family also dominated the domain of Qohestan and played an important role during the decline of the Samanids in the direction of their destruction. This article deals with the analysis of the role of the Simjurids family in the decline of the Samanids with a compiled library method. The results of the research show that the tendency of the Simjurids to have a hard partnership in the government and to rely on the relatively large vassalage of Qohestan prevented the concentration of power for the Samanids and provided one of the main factors for the downfall of this dynasty.