why I get a cube with my model ?
Possibly because your model has non-manifold edges or other similar issues which prevent the Cell Fracture add-on to work properly. You can check for non-manifold edges in the model to be fractured by going in Edit Mode, activate the Edge Select Mode then deselect all (A) and type Ctrl+Shift+Alt+M.
To experiment it load Suzanne and try to cell fracture it; it won't work because of the previously described problem. Now, follow the steps above and you'll see where the problems are. When you have corrected them, try again the Cell Fracture and it should work fine. You may also search for video tutorials online on Cell Fracture for more information on its usage.