Abstract: Supporting and retaining coal pillar is one of the important means to improve the stability of internal waste dump with weak basement. To solve the problem of designing morphological parameters of supporting and retaining coal pillar on internal waste dump with weak basement,and to achieve the safe production and economic benefits of the mine,the mechanical effects and shape design methods were studied. Based on the mechanical model,the three-dimen- sional force state analysis was carried out. The supporting and retaining coal pillar was regarded as a fixed beam under the dual action of the static earth pressure of internal waste dump and the shear force at the bottom interface. The Mo- hr-Coulomb criterion was used as the instability criterion,authors derived the expression of three-dimensional supporting and retaining effect of coal pillar. Based on the type of mechanical genesis and influencing factors,a two-dimen- sional equivalent method for the supporting and retaining effect of coal pillars was proposed and the expression of cd and φd were derived. In theory,the influence law of morphological parameters on the supporting and retaining effect was revealed. The method was verified by numerical simulation the internal dump Shengli East No. 2 open-pit mine. The results showed that cd and φd are positively inversely proportional to their top width a and lower bottom angles β and ω, positive correlat with the height H,and negative correlat with the length d The stability coefficient Fs of the in- ternal dump of Shengli East No. 2 open-pit mine increases with the increase of a,and increases linearly with H The op- timal size of the coal pillar is 28 m and 43 m. Compared with the traditional two-dimensional analysis results,more than 75 600 m3 of raw coal can be produced,and the economic benefit is remarkable. The achievement optimizes the morphological parameters of supporting and retaining coal pillar to make up for the inadequacy of excessive coal losses caused by the traditional and to achieves a maximum economic benefit in mining operation.