Fire-retardant polymer electrolytes are key materials for safer operation of lithium secondary batteries and other large-size batteries than the batteries with organic liquid electrolyte. Solid polymer electrolyte systems with fire-retardant polymer matrixes have been investigated in only few cases. We prepare an oxetane derivative with dimethyl phosphate ester group (DPOX). The cross-linked poly(oxetane) matrixes for solid polymer electrolytes are prepared from DPOX and oxetane-based cross-linker (DDOE). DDOE has two oxetane rings at both of the ends of oligo-ethylene oxide chain. Conductivities in the range of 10−5 Scm−1 have been obtained for self-extinguishing, ion-conductive cross-linked poly(oxetane) matrix with DPOX and DDOE monomers. Polarization behavior of a lithium electrode in the cross-linked poly(oxetane)-based electrolyte films is also investigated. The polarization curves show some hysteresis character with potential steps, however, deposition and stripping of lithium on the lithium electrode are observed.