In this work we synthesize a series of plasticizers and a crosslinker based on a multi-armed oligo(ethyleneoxy) phosphate and carry out in-situ radical polymerization of the precursor solution containing the plasticizers and crosslinker to produce semi-interpenetrating polymer network (semi-IPN) solid polymer electrolytes (SPEs). A high-quality free-standing film is obtained with a tensile strength as high as 1.2 MPa. Several factors are investigated to optimize the ionic conductivity of the solid polymer electrolytes such as the length of ethylene oxide units in the plasticizers, the concentration of lithium salts, the content of the plasticizers, and the different types of lithium salts. The maximum ionic conductivity of the SPEs is found to reach 5.0 × 10−4 S cm−1 at 30 °C, together with the wide electrochemical stability window of above 5.0 V and the columbic efficiency more than 70% in reversible lithium plating–stripping cycles, making phosphate-based semi-IPN SPEs a promising candidate for application in solid-state lithium secondary batteries.