Due to the dislocation distribution of wind resources and load centers in China, large-scale wind power stations usually need long-distance transmission to connect with the load center, which leads to the weakening of the power grid strength at the access point of wind power stations and frequent short-circuit faults. In this paper, a small signal state space model of flux-controlled grid-forming DFIG grid-connected system is constructed for the continuous condition of symmetrical fault in weak grid scenario, and the oscillation mode analysis of the system is carried out. Based on the fault ride-through requirements and electromagnetic equations of wind turbines, the calculation method of the balanced operating point of DFIG during symmetrical fault duration is derived. Then, based on the system characteristic root trajectory and the participation factor matrix, the small-disturbance stability of the system under different fault conditions and converter control parameters is analyzed, and the influence of each dynamic link on the small-disturbance stability of the system is explored. Finally, the accuracy of the theoretical analysis results in this paper is verified by simulation analysis and comparison, which provides guidance for parameter optimization of DFIG during fault duration.