Objective: Magnetic drug targeting (MDT) has attracted a lot of attention in recent years as a treatment to reduce side effects and improve efficacy. To realize successful MDT, a magnet system that can create suitable magnetic field is necessary. However, the existing technology has some shortcomings such as low targeting efficiency and unsatisfactory targeting effect. The method of using time-varying magnetic field proposed in this paper provides a new solution to this problem. Methods: In this work, a permanent magnet system providing rotational magnetic field (a time-varying magnetic field) was designed and constructed. Focusing experiments of $Fe_3 O_4$ particles were carried out in this system, and the mechanism of the focusing was discussed via a simplified model. Results: By using the rotational magnetic field, superparamagnetic $Fe_3 O_4$ particles can be successfully driven to the designated site within a short time. Our work showed that the time-varying magnetic field can drive the superparamagnetic particles to a focusing site, which is not possible under the same magnetic field without rotation, and the aggregation speed and effect are better than those under static field. Conclusion and Significance: Our results indicate that time-varying magnetic field can be a good choice for designing magnet systems for MDT. Moreover, the idea of using time-varying magnetic field for focusing superparamagnetic particles may be applicable in non-contact processings or manipulations of paramagnetic, superparamagnetic, or even ferromagnetic materials or objects in preparation of novel materials, robotic control of objects, and so on.