In order to obtain the mode Ⅰ stress intensity factor with high accuracy,a method for determi-ning mode Ⅰ stress intensity factor based on symplectic geometry method is proposed.By constructing a fan-shaped region at the crack tip,separating the full state vector of the crack tip under the symplectic ge-ometry method,combined with the displacement field at the crack tip,the stress intensity factor is calculat-ed.Based on the theoretical solution of the displacement field around the mode Ⅰ crack tip,the effects of the number of characteristic points,the size of center angle and radius length on the accuracy of stress in-tensity factor are studied.The results show that the number of characteristic points has a great influence on the accuracy of solving mode Ⅰ stress intensity factor by this method.As the number of characteristic points is greater than 37,the relative error tends to 0 stably;the sector center angle has little effect on the accura-cy of stress intensity factor,but under the same accuracy requirements,different sector center angles need different number of feature points;as the sector radius is too small,the solution of stress intensity factor is unstable and the relative error is large due to the influence of the plastic region at the crack tip in the frac-ture process.By comparing the relative errors,it can be found that the accuracy of the mode Ⅰ stress inten-sity factor calculated by the proposed method is high,and the stress intensity factor can be accurately ob-tained only by obtaining the displacement field around the crack tip.It solves the problem that the previous solution system has complex steps in the case of complex stress,resulting in the inability to accurately ob-tain the mode Ⅰ stress intensity factor.