A filter is provided, and the filter includes two mutually coupled slow-wave resonators. Each resonator includes a coplanar waveguide (CPW) transmission line, a tapered CPW transmission line, and a ground stub, and can generate a slow-wave feature to push a high-order harmonic wave of a baseband signal to a high frequency, so as to implement a wide stopband feature. In addition, a slow-wave effect is used to properly design a size of a filter, to reduce an entire area of the filter and reduce costs. Moreover, two resonators are coupled, to enhance passband performance of the filter, increase bandwidth, increase in-passband flatness, and reduce an insertion loss.