The high durability of low-k value is the key parameter for the low dielectric materials especially used under humid conditions, because the k value is very sensitive to moisture. Here, to decrease the dielectric constant and improve the water resistance, a series of novel co-polyimides are successfully prepared based on cross-linkable diamine monomer by a two-step pathway combining polymerization and crosslinking reaction. It is intriguingly found that with the gradual increase content of cross-linkable group, the dielectric properties, water resistance as well as thermal and mechanical properties of those co-polyimides can be finely tuned. After cross-linking, the dielectric properties are significantly enhanced and the film surface changed from hydrophilic (78.7-89.2 degrees) to hydrophobic (93.0-104.2 degrees), which endow the cross-linked films with superior water resistance and high durability of low-k value. The best performance is obtained for cross-linked co-polyimide (CL-Co-PI-4) containing 30% cross-linkable group. The CL-Co-PI-4 exhibits low dielectric constant of 2.32 and dielectric loss of 0.016 at 1 MHz. Excitingly, the CL-Co-PI-4 demonstrates an extremely low water uptake of 0.051 +/- 0.010%, which represents the best water resistance for the reported polyimides. After exposing to different humidity conditions for 12 h, the increasing percentage of k value is very low and below 0.84%. The CL-Co-PI-4 still keep its k value below 2.35 after equilibrating at 75% R.H. for 14 days. The excellent moisture resistance and overall performance make the CL-Co-PI-4 a good candidate for dielectric material under both dry and humid conditions.