The inerter can replace the mass block and realize the dynamic characteristic of large inertia body with small mass, thus greatly enriching the design theory of mechanical vibration network. To achieve the same inertance, the larger the inertia-mass ratio is, the lighter the mass of the flywheel is, and the advantage is more obvious. In order to improve the inertia-mass ratio of ball screw type inerter, a new design of ball screw type inerter with planetary flywheel was put forward. By the mechanical model of inerter, the influence of structure parameters on the inertia-mass ratio of flywheel was researched. Result showed that the inertia-mass ratio could be improved by three methods: increasing the number or thickness of the planet gears, reducing the pin diameter, which increased the proportion of planet gear rotation inertia in the total inertia of flywheel. The maximum inertia-mass ratio could be obtained by the optimal gear ratio of planet gear to ring gear, which was calculated by the number and thickness of planet wheel and the pin diameter. The mechanical properties of planetary flywheel inerter were compared with those of traditional inerter. Analysis of result indicated that the inertia-mass ratio of planetary flywheel inerter was larger than that of traditional inerter with the same diameter of flywheel. But for the same radial size, the planetary flywheel was thicker than the traditional-single flywheel. Therefore, the planetary flywheel inerter was suitable for the weight sensitive, and the axial space-rich application occasions. The theoretical analysis was verified by experiment at last.