In view of the situation that Sijiaying open-pit does not have the condition of plane boundary extension and needs to have a slope boundary pillar after underground mining, the stability of slope boundary pillar after underground mining is studied by using the gravity increase method, and the evolution characteristics and law of the stress field, displacement field and plastic zone of slope boundary pillar variation with the gravity acceleration are obtained. The results indicate that (1) under the initial gravity acceleration, a 40m thick slope boundary pillar can ensure the safety of underground mining and avoid the instability of the open-pit slope such as collapse and sliding phenomenon; (2) with the increase of gravity acceleration, the progressive instability failure of slope boundary pillar can be divided into four stages: stable, slight deformation, serious deformation and instability failure. The western slope of the open-pit within the exploration line N22~N26 in the mine is the most dangerous area where the slope occurs collapsing, slipping and other instability phenomena after the mine is transferred to underground mining; (3) affected by the unloading of underground mining, the maximum principal stress of slope boundary pillar is basically within 30MPa, while the minimum principal stress and its distribution range increase greatly. The value of tensile stress reaches more than 3MPa, which exceeds the tensile strength of rock mass and are easy to produce tensile failure of the pillar; (4) the displacement variation and the plastic zone expansion of the slope boundary pillar is consistent with the characteristics of the tensile stress, which extend from the slope foot to the top of the slope on the west side of the open-pit. When the gravity acceleration increases to 2 times the initial one, the plastic zone extends upward to the top of the slope in the slope boundary pillar on the west side within the exploration line of N22~N26, and the maximum displacement reaches 1.5m, resulting in the overall instability of the open-pit slope. The research findings play an important guiding role in the prevention and control of slope instability after the mine is transferred to underground mining.