Swarm, the fourth "Earth Explorer" mission of the ESA, is mainly to determine the geomagnetic field. One of the key issues in using Swarm's data to construct the magnetic field model for each layer of the earth is to accurately separate geomagnetic signals from different field sources. In this paper, based on quiet magnetic period observations data of the ESA's Swarm satellites, the external source fields are processed with finite impulse response (FIR) high-pass filtering along the satellite tracks, the length of the filter window is discussed. In addition, the feasibility of this method is proven by the MIO_SHA_2D and MMA_SHA_2C, which are ionospheric and magnetospheric magnetic field models published by the ESA in 2016. The results indicate that the outcome of FIR filter for long wave signals of the external source fields well agree with the model correction results. When the passband and stopband wavelengths are set to 1 200 km and 3 000 km, respectively, if we set the length of the filter to 87-129 orders, it can achieve a better balance of the filtering accuracy in the horizontal and radial.