【Purposes】 It aims to analyze the effect of the size and shape of silicon quantum dots on their binding energy. 【Methods】 The cohesive energy of silicon quantum dots with different sizes and shapes is calculated by using the method of density functional theory first principles. 【Findings】 The calculation results show that the cohesive energy of silicon quantum dots basically exhibits shows an increasing trend with the increase of atom number in the system, which is consistent with the size effect of the cohesive energy of nanomaterials. However, this size effect is also influenced by shape. By introducing a shape factor λ, it is found that the cohesive energy of tetrahedron (TH) silicon quantum dots is always smaller than that of tetrahedron-centered symmetric (TC) silicon quantum dots at equal sizes, and the cohesive energy of both is also smaller than that of octahedron (OT) silicon quantum dots. In some silicon quantum dots, the cohesive energy of silicon quantum dots with smaller atomic number is larger than that of silicon quantum dots with larger atomic number because the silicon quantum dots with smaller atomic number have larger shape factor. In addition, the ratio of hydrogen atoms to silicon atoms in the silicon quantum dot system, i.e., n(H)/n(Si), can reflect the size and shape parameters of silicon quantum dots at the same time. The linear relationship between the cohesive energy of silicon quantum dots and the ratio of hydrogen atoms to silicon atoms further proves the size and shape effects of silicon quantum dots.