For the power quality control and relay protection， it is very important of rapid， correct and practical detection of the negative sequence fundamental current. After analyzing the selective frequency characteristic of an amplitude integration signal， this paper proposes an extractor with a novel detecting method of the negative sequence fundamental current. This detecting method needs neither a phase locked loop（PLL） nor a low pass filter（LPF）， and can omit some complex calculations for the coordinate transforms. Results from the simulations and experiments all verify the perfect performances of the shunt APF which employs this novel detecting method.