Due to the construction of Haraz Dam, about 9 km of Haraz Road is within the dam reservoir; therefore, it is necessary to construct an alternative route at a higher level. In this manuscript, the stability of the rock slopes of the tunnel portal in the Pern Valley area located on the alternative road route overlooking Haraz Dam was analyzed using kinematic, experimental, and numerical methods, and its numerical modeling was performed with 3DEC software, and the proposed retention system was examined. For this purpose, first, the probability of all three types of plate, wedge, and overturning failures in the upstream and downstream slopes of the road was determined by kinematic analysis, and then, by analyzing the stability of the slopes with the SMR experimental method, it was determined that the rock mass is classified as very bad in some cases and a retention system is required in all cases. Numerical modeling of slope stability using 3DEC software, the maximum displacement of the slope block was estimated to be 8.42 cm and the safety factor of the studied slope was calculated to be 1.39, which is lower than the allowable safety factor for medium and long-term stone walls (1.5). According to the results of numerical analysis and SMR results, the best proposed retention system is to install rock bolts locally and spray concrete with a thickness of 10 cm. Based on the results of numerical modeling in 3DEC, by implementing the proposed retention system of rock bolts and shotcrete, the maximum displacement increases to 1.053 cm and the slope safety factor increases to 1.55.