An improved phase retrieval method with dual-plane holograms for slightly off-axis digital holography (DH) is proposed. By applying the Hilbert transform (HT) twice on defocused hologram, the DC term can be effectively eliminated while preserving the original object information. The phase of tested sample can be retrieved from two transformed holograms with only a straightforward algebraic equation. The derivation of algebraic equation and characteristic analysis of HT on hologram are discussed in details. Our method offers a viable solution for quantitative phase imaging in slightly off-axis DH system, eliminating the need for complex spectrum filtering, phase-shifting devices, and iterative operation. Numerical simulation and experiment results of microlens array and phase plate demonstrate the validity of proposed method.