A large area of tensile stress in the middle and lower part of the upstream of a stone-masonry arch dam exceeds the standard and needs to be strengthened. Based on two local reinforcement schemes of a masonry dam, the contact element was used to simulate the joint characteristics of the new and old dam bodies, and the stress and deformation characteristics of the joint surface under five standard working conditions were analyzed. The results show that after reinforcement, the opening and loading characteristics of the joints of the two schemes are similar, and the side pier of the rib pier scheme has asymmetrical opening and loading phenomenon. Under the condition of high water level, the reinforced structure mainly supports the load at the upper part, and the stress at the lower part is small and may open. Under the condition of low water level, the stress level of the joint surface is low and the reinforced structure does not bear the load. Under the condition of temperature drop after reinforcement, the masonry dam body rotates and deforms with the upper part of the reinforced structure as the fulcrum, and the joint surface will open in a large area under the condition of low water level. Under the condition of temperature rise, the dam body shifts upstream when the water level is low, resulting in a symmetrical and smooth parabolic clearance distribution on the horizontal direction.