An improved affine projection algorithm for hearing aids is proposed to increase the convergence speed and reduce the misalignmengt. The algorithm establishes a new nonlinear function between the step-size and the estimation error. The step-size of the proposed algorithm is adjusted automatically according to the change of the estimation error, which leads to high convergence speed and low misalignment. In order to improve the accuracy of estimating the power of error, a rule for selecting forgetting factor is proposed. Mathematical analysis provides theoretical basis for its outstanding capacity of converging to the objective system. The simulation results show that the proposed algorithm achieves faster convergence speed and lower final misalignment compared with traditional adaptive methods and fixed step-size affine projection algorithm.