Decoupled double synchronous reference frame phase locked loop(DDSRF-PLL) is widely used for asymmetric grid synchronization. However, the stability issue due to the interaction between DDSRF and PLL have not been explored clearly. In this paper, the small-signal model for two typical implementation forms of DDSRF is developed in polar frame respectively. Then, the differences in coupling characteristic and stability region of two DDSRF structures are further discussed. It’s revealed that the structure with placing low pass filter(LPF) in the feedback path significantly reduce the coupling effect between DDSRF and PLL, which improves system stability margin. Finally, the theoretical analysis is well validated by simulation results.