The invention relates to POM (polyoxymethylene)-PLA (polylactic acid)-poly(p-dioxanone) ternary composite fibers and a preparation method thereof. The composite fibers comprise components including POM, PLA and poly(p-dioxanone), and the mass ratio of POM to PLA to poly(p-dioxanone) is (60-80):(12-30):(3-15). The preparation method comprises steps as follows: the three polymers are dried to reach constant weights, extruded by a screw, pass a pre-filter, a box body and an assembly, cooled in a spinning channel and wound, as-spun fibers are subjected to backend heat drawing and heat setting, and the composite fibers are obtained. The composite fibers have good mechanical performance, heat resistance and proper degradability and have a good application prospect.