Virtual studio is a new application of virtual reality in TV program production. Unlimited blue-box technique is the way to cover the scenery out of blue-box with an external chroma-key signal. It extends the movement of the camera in virtual studio. A universal unlimited blue-box technique is proposed based on the perspective projection. It can be applied to all kinds of blue-box structures and varied camera movements. The experiment result demonstrates that this technique is accurate and robust, and is sufficient to the requirements of virtual studio.