Low dielectric constant (D_k) polyimide polymer materials are one of most attractive materials for their importance in microelectronic industries, such as high speed communication field. Herein, a novel benzocyclobutene (BCB) and trifluoromethyl functionalized diamine monomer (BFDA) was synthesized and used to prepare a series of co-polyimides with different BCB content (PI-BFs). Both trifluoromethyl and BCB structure can facilitate to low down D_k and dissipation factor (D_f). For the cured PI-BFs, dielectric property test showed that the dielectric constant decreased significantly with the increase of BCB content, and the dielectric constant of PI-BF_40% could reach 2.72 at 1 MHz. Besides, BCB can also contribute to improve glass transition temperature and initial storage modulus of PI-BFs. The Tg of PI-BF_40% was 360 ^oC, and the initial storage modulus reached 2.83 GPa. Meanwhile, TMA test indicates that the post-curing by BCB group contributes to enhance the thermo-size stability. Moreover, PI-BFs show excellent thermal stability (T_d5>499 ^oC), good transparency and low water uptake.