A family of oligo(ethyleneoxy)cyclotriphosphazenes was synthesized as low molecular (or oligomeric) ionic conductors (plasticizer) and incorporated with lithium salt into semi-interpenetrating network (semi-IPN) solid polymer electrolytes (SPEs) for the improvement of ionic conductivity. A novel semi-IPN SPE was prepared by in situ radical polymerization of precursor solution containing a new cross-linker PA6 and plasticizers PMEn (n = 2, 4, 6, 8) based on cyclotriphosphazenes, which exhibited high ionic conductivity of ca. 10(-4) S/cm at room temperature with significantly enhanced dimensional stability. The effect of the number of ethyleneoxy (EO) units in plasticizers, lithium salt concentration, and the content of the plasticizers on ionic conductivity of SPEs was investigated. The thermodynamic behaviors of the cyclotriphosphazene plasticizers were also analyzed to figure out the contribution of plasticizers to the improvements of electrochemical properties in the semi-IPN SPEs. Cyclic voltammetry and chronopotentiometry were conducted to confirm the electrochemical stability of this semi-IPN SPE system.