The utility model relates to a device may realize a double gradient drilling which is characterized in that the device comprises a drill stem, a drilling device which is composed of an injection device which may be separated in drilling, a subsurface safety valve and a bit, as well as a mud handling device. The injection device which may be separated in drilling comprises a column shaped framework which is positioned in a drill hole through surrounding supporting devices. A sleeve is rotatably arranged in the column shaped framework, with the upper end and the lower end connected with the drill stem respectively. A conic shell is positioned outside of the sleeve and fixed in the column shaped framework. A light microballoon injection module and a mud injection module are respectively arranged at the top and the bottom of the conic shell and fixed on the sleeve. A plurality of through holes is correspondingly arranged on the sleeve and the circumferential direction of the conic shell. A plurality of light microballoon injection channels are correspondingly arranged on the top of the conic shell, the light microballoon injection module and the column shaped framework respectively. The bottom of the conic shell, the mud injection model and the sleeve are correspondingly provided with a plurality of mud injection channels respectively. The utility model has the advantages of simple structure and low operation cost and may be broadly applied to the drilling operations of deep wells.