[Objective] To address the problem that traditional single grid-following (GFL) or grid-forming (GFM) control is difficult to adapt to under alternating strong and weak grid conditions in doubly-high power systems, a grid impedance identification-based GFL/GFM switching control strategy is proposed. [Methods] Grid strength was monitored and the short-circuit ratio (SCR) was determined by using a grid impedance identification technique based on specific harmonic extraction. A hysteresis switching loop was set up: GFM control was adopted when SCR was less than 2, GFL control was adopted when SCR was greater than 3, and the original control mode was maintained within 2≤SCR≤3 to avoid frequent switching in the critical interval. Transient impacts during mode switching were eliminated through a steady-state operating point pre-matching strategy, which compensated the power reference values and initial virtual excitation values. Finally, simulation verification was carried out using Matlab/Simulink. [Results] During the alternating variation of strong and weak grid conditions, the proposed strategy accomplished bidirectional switching between GFM and GFL control based on online identification results. At the switching instant, no significant step occurred in the point of common coupling voltage, and the current only exhibited short-term disturbance before rapidly recovering to steady state, with no sustained oscillation in active and reactive power. Comparison results with different switching methods further indicated that steady-state operating point pre-matching mitigated current impacts during the switching process and rendered the mode transition smoother. [Conclusion] This strategy integrates the technical advantages of GFL and GFM control. It can realize GFL/GFM switching control according to changes in grid strength, and reduces the voltage, current, and power impacts during mode switching through steady-state operating point pre-matching, thereby improving the operational adaptability and switching stability of the power conversion system under dynamically varying strong and weak grid conditions.