Photovoltaic Islanding detection is a important function for grid-connected inverters. It is difficult to detect islanding when there is a power balance between the inverter and the load. The active frequency drift with positive feedback method (AFDPF) is able to detect the islanding by disturbing the frequency of inverter output current under the situation of power balance. This paper is dedicated to analyze the principle of AFDPF method and the parameters selection rules. The non-detection zone (NDZ) of AFDPF is also presented. The analysis results are verified by the simulation in MATLAB/Simulink. According to the technical specification of IEEE.Std.929-2000, the validity of the analysis is supported by the experimental results.