A new type of comb cross-linked polyurethane/acrylate polymer was designed. The polymer has sparse network structure with many long comb molecule chains. A new solid polymer electrolyte (SPE) was prepared based on the polymer. The salt in the solid polymer electrolytes has different existent states with different salt contents. With increase of salt concentration, the ion pairing gradually becomes important existent form of salt, and T_g value of the SPE increases. At the same time, ionic conductivity increases rapidly. It is possible to design novel solid polymer electrolytes with high ionic conductivity to meet practical application by comb cross-linked polymer with high salt content.