Low density polyethylene and linear low density polyethylene were used as basic materials and ammonium polyphosphate was added into them to prepare fire-retardant system. The molding technology parameters and fire-retardant properties of the fire-retardant system were studied. The results showed that when ammonium polyphosphate content is 30%, the fire-retardant level can reach FV-0; the rotational speed of main screws is 28~37 r/min, the pulling speed is 2.8~5.5 m/min; when molding dumbbell samples, the cylinder temperature is between 185~200 ℃, the mold temperature is between 91~105 ℃, the warm-up time is from 5 to 10 min and molding time is from 12 to 21 s; when molding rectangular samples, the cylinder temperature is between 185~190 ℃, the mold temperature is between 85~110 ℃, the warm-up time is from 5 to 10 min and molding time is from 8 to 13 s.