Aiming at the problem that a new generation of sub-caliber ammunition with sabot, which will be used in a 100mm naval gun, and which requires the retractor expansion chamber to have enough expansion volume, and also ensures that the sabot of the sub-caliber projectile can't collide with the muzzle device, the main dimensional parameters of the muzzle retractor and the corresponding parameter variation ranges have been determined with the reference to the standards of the weapon industry and the actual working conditions. Meanwhile, two methods of theoretical calculation analysis and fluid simulation analysis are used to analyze the retracting efficiency of the muzzle retractor with different lengths of impact baffles in different numbers of chambers, and a muzzle retractor suitable for this 100 mm gun is designed. The results show that the retractor with 4 chambers reaches the maximum retraction efficiency near 210 mm impact baffle length, while the retractors with 5 and 6 chambers have the maximum retraction efficiency near 200 mm. The reason is that when the number of chambers is too small and the impact baffle is too short, the airflow is more likely to overflow in advance, while the number of chambers is larger and the airflow is decelerated in the front chambers, the possibility of overflowing in advance in the chambers at the back of the chambers is relatively reduced, so when the number of chambers is large, the impact baffle can be slightly shorter to achieve the effect of relatively high retracting efficiency.