The view-angle of rainbow holograms which are recorded using a pre-existent slit or a pre-existent slit with mirrors is analyzed in detail. Then a simple method using a pre-existent slit with inclined mirrors is suggested. This method can extend the view-angle of the rainbow hologram further, while keeping a high energy efficiency of the rainbow hologram. The best position of the mirrors and the possible biggest view-angle of the hologram is studied and the relative formulas are given. Requirements and restriction of using the three methods are considered and their biggest view-angles are compared. A concept on efficiency reconstruction slit is proposed, so as to distinguish it from the normal reconstruction slit.