A fast reliability-guided phase unwrapping algorithm for digital holography is proposed. The value of reliability is nonlinearly quantized to form a quality map. The phase is unwrapped reliably and fast by combining the nonlinear quality map with look-up table operation. The residues are searched in the wrapped phase map, and the maximum intensity of residues is set as threshold. The measured region is segmented to the reliable region and the doubtful region by the threshold. A nonlinear quality map is derived by setting the reliability of reliable region to maximum and setting those of the doubtful region to quantized value of intensity. Flood-fill algorithm using look-up table is implemented with the nonlinear quality map to unwrap the phase map. In experiments, the speed of proposed algorithm is about 68 times faster than that of conventional flood-fill algorithm and about 3 times faster than that of branch cut algorithm. The experimental results demonstrate that the speed of proposed algorithm is significantly increased and the quality of unwrapped phase is good as well.