By setting up diagonals applied with prestress, the miter gate in Three Georges Project permanent lock can raise the torsion rigidity. Based on three dimensional finite element method, the stresses and displacements of the miter gate applied by every single load are calculated. The 14 － bar optimization model and a simplified 4 － bar optimization model for calculating the diagonal prestress are established. It is deduced from the optimization models: ① When one upper diagonal is tensioned, only the stresses of the upper diagonals are changed, and those of the lower diagonals not changed almost; When one lower diagonal is tensioned ,only the stresses of the lower diagonals are changed, and those of the upper diagonals not changed almost. ② As the stresses of the upper primary diagonals σ=1 MPa simultaneously, then the stresses of the upper secondary diagonals σ=0.7 MPa； As the stresses of the lower primary diagonals σ=1 MPa simultaneously,then the stresses of the lower secondary diagonals σ=0.5 MPa. Some groups of prestress are calculated, it can be used for the adjustment of the diagonal prestress.