Digital hearing aids are the main ways to treat hearing and speech residual diseases. In recent years, as associated digital signal processing (DSP) technologies are widely researched, the control, modification and enhancement of the voice signals and filtering techniques, which are applied to digital hearing aids, are significantly developed. From the aspects of loudness compensation, noise suppression and echo cancellation, recent studies of the algorithms and implementations at home and abroad, which are reviewed according to the current structures of the mainstream digital hearing aids systems. After that, According to the problems and different solutions for different strategies，corresponding comparative analysis and evaluations are carried out. Finally, unresolved issues of algorithm program are discussed at the present stage for digital hearing aids, then the future direction and development of related technologies are predicted with the prospect.