Abstract: The traditional algorithm is widely utilized in power flow calculations of distribution network due to its clear iterative logic and favorable convergence. However, its calculation depends on the balanced nodes, which makes it unable to address the issue of the islanded microgrid without balanced nodes under droop control. In this paper, we propose an affine power flow algorithm for islanded microgrid based on double-layer forward-backward substitution. First, an affine power flow model of distributed power supply and load under droop control is established. Secondly, a double-layer iterative forward-backward substitution algorithm is employed to calculate the affine power flow. In this algorithm, the outer layer iteratively corrects the system frequency and the root node voltage, while the inner layer uses the forward-backward substitution method to calculate the complex affine voltage values of the remaining nodes. Finally, according to the system frequency and the different iterative states of the root node voltage, the affine power flow algorithms under three different iterative methods are summarized in detail, and their convergence performance is compared and analyzed. The results of the example demonstrate that the double-layer affine algorithm proposed in this paper can significantly enhance computational efficiency while maintaining the conservation and completeness of the calculation. The system frequency and the root node voltage are set in the outer layer simultaneously, leading to optimal convergence performance during iteration.