A reformed method for detecting microangular displacement was proposed according to the characteristic of the half-wavelength GRIN lens. The transfer function from the microangular displacement to the coupling efficiency was deduced by analyzing the transmission of the beam and the geometrical parameter of the model. Considering the limit to measure range of the model,a feasible method was also proposed. The result of the research indicates that this model not only has a wide measure range,but also has a high precision with good linearity.