This paper presents an improved direct adaptive fuzzy controller (IDAFC) for single-phase active power filter (SAPF). Although fuzzy control has many successful applications in improvement of power quality, many fuzzy control strategies have no systematic stability analysis and controller design. Systematic stability analysis is performed in the design of proposed IDAFC. Firstly, the principle of SAPF is introduced. Then, based on the fuzzy basis functions and Lyapunov stability, the design of IDAFC is proposed. Finally, simulations show that IDAFC not only has better dynamic performance and similar filtering performance compared with conversational fuzzy controller, but also has robustness under filter parameters variation.