Numerical simulation is used to study the optimal values of the balancing hole number and the diameter of hole opening circle of a balanced orifice plate. It introduces the method and procedure to obtain optimal balancing hole number and circle diameter ratio are presented for the case of pipe diameter D= 100 mm and equivalent diameter ratio of β = 0. 6. It is found that the optimal hole number is N= 10, after comparing the discharge coefficient, pressure loss and momentum difference for different hole numbers with a fixed circle diameter ratio of K= 0. 67, and the optimum circle diameter ratio is K= 0. 67, after comparing the above-mentioned evaluation indexes for different circle diameters with a fixed balancing hole number of N = 10. The optimal mechanical parameters are given for various combinations of five pipe sizes within DN50 to DN1000 and six equivalent diameter ratios. The optimal balancing hole number is 8 or 10, and the optimal center-circle diameter ratio K is between 0. 64 and 0. 73 ( in most cases, it is between 0. 66 and 0. 70). In the paper, the concept of momentum difference is proposed to evaluate the performance of a balanced orifice plate, and it is proved that ΔM reaching to the minimum value can be used as an auxiliary criterion for achieving an optimal mechanical parameter. The results of the study are of reference value for the mechanical design of a balanced orifice plate.