Based on cascaded H-bridge inverter, multilevel modulation technology is applied to system of STATCOM. The key parameters of H-bridge module are given and designed in this paper. Meanwhile, this paper puts traditional feedback decoupling control method and APF control technology into STATCOM, a control strategy of reactive power compensation and harmonics suppression is proposed to 1140V cascaded STATCOM .This device can optimize the grid power quality. Under this theory and technique, test prototype of 1140V cascaded STATCOM is built. Test shows the prototype can complete reactive and harmonics compensation and has a good compensation effect.