An anti-seismic device is adapted to absorb the vibration energy of a structure, and comprises a plurality of vibration-reducing units, each of which includes a mass, a hydraulic cylinder, and a resilient element. The hydraulic cylinder has an outer end that is connected fixedly to the structure, and an inner end that is connected fixedly to the mass. The resilient element has an outer end that is connected to the structure, and an inner end that is connected to the mass. When the structure is subjected to vibration, the masses can move reciprocally relative to the structure at different vibration frequencies. The hydraulic cylinders are arranged substantially in parallel.