Abstract: An effective way for full-power wind turbine converters to improve the adaptability of power grid strength is to switch smoothly between the grid-following mode and grid-forming mode. The scheme of DC voltage synchronization takes DC voltage as the control target, which is easy to produce serious transient shock of DC voltage during switching, leading to misoperation of DC chopper and reduction in the service life of hardware. Therefore, firstly, the smooth mode switching principle is determined by vector analysis method, and the causes of transient shock of DC voltage are analyzed by combining the transfer function of switching disturbance to DC voltage and switching disturbance itself. Then, based on the analysis, a direct switching scheme is proposed to solve the step disturbance problem of steady-state operating point, but there is still a random disturbance of current given. A delay switching scheme is proposed to minimize the given current disturbance by delaying switching signals. RT-LAB hardware-in-the-loop experiments show that the direct switching scheme can greatly improve the transient shock of DC voltage, while the delay switching scheme can further ensure the smooth switching process without fluctuation.