This letter presents a compact, low-loss flexible filter based on a double-interdigital coupling stepped-impedance resonator (SIR) and through-polyimide via (TPV) technology. With a fractional bandwidth (FBW) of 12.5% at 2.5 GHz, the filter achieves a low insertion loss of 1.8 dB in the flat state and only 1.95 dB under bending. Its electrical performance remains highly stable under deformation, while the structure attains a 40% size reduction over the state-of-the-art designs, offering a promising solution for wearable wireless systems demanding flexibility and spectral selectivity.