The Huangchengtai retaining wall is an important part of Shimao Huangchengtai and the key to ensuring the safety of Huangchengtai. At present, the mortar joint material used for the retaining wall is seriously weathered and has a certain inclination deformation, so it is urgent to evaluate its stability. Through a theoretical analysis, a formula suitable for calculating the stability of the retaining wall of Huangchengtai was derived. The accuracy and applicability of the formula were verified by laboratory tests. The influences of the ratio of height to width, the mortar joint strength and the vertical external load on the stability of the retaining wall were analyzed. The results show that the retaining wall was a gravity retaining wall with a loose interior, and that there were two types of failure: sliding and overturning along the internal cracks. The overturning angle was related to many factors, such as the wall height and mortar joint strength. When the mortar joint strengths were the same, the stability coefficient at the bottom of the retaining wall was the smallest—the most prone to overturning. The stability of the retaining wall decreased with the increase of height-width ratio and vertical external load, and with the decrease of mortar joint strength. This study could provide a scientific basis for the stability assessment and reinforcement of the retaining wall of Shimao Huangchengtai.