In this paper, the effect of poly (ethylene oxide) (PEO) as an additive on the structure and properties of poly (m-phenylene dimethylene terephthalamide) (PMIA) fibers obtained by wet spinning was investigated. The tensile strength of the composite fibers was substantially enhanced compared to the pure PMIA fiber. This was due to the fact that the addition of PEO weakens the hydrogen bonding between PMIA molecular chains resulting in an improved orientation of the composite fibers. It was found that the optimum PEO addition was 2% and the tensile strength of the composite fiber was 4.74 cN/dtex, which was 76% higher compared to the pure PMIA fiber. However, the heat resistance and flame retardancy of the composite fibers were basically unchanged compared to the pure PMIA fiber. The modification method is simple, with low raw material cost and good stability, and has not only good academic value but also excellent industrial value.