A novel network polymer electrolyte based on the polymethylsiloxane system has been designed and synthesized. In the network polymers, different numbers of PEO chains with various molecular weights were introduced onto the networks as immobile internal plasticizer through a hydrosilylation reaction. The presence of internal plasticizers pendant on the network backbones can lower the glass transition temperatures (T g) of the electrolytes, make the lithium salt easily dissolved and accelerate the migration of free lithium ions migration. The ambient ionic conductivity (σ RT) was significantly enhanced compared with that of network polymer electrolytes without internal plasticizers. The optimum ionic conductivity was 10−4.1 S cm−1 at room temperature and 10−3.0 S cm−1 at 80°C.