We propose a method to improve relative laser power stability using a passive photosensitive sunglass lens, which is a commercially available off-the-shelf product. We present a theoretical analysis and identify factors that affect the optimal working state of the lens. A relative laser power stability of 3.3 × 10−5 at 1 s is experimentally achieved, which is more than three times that of 1.2 × 10−4 at 1 s, acquired without power stabilization. This method does not require any active driving device, thereby significantly reducing the complexity and cost of the system, making it suitable for broad applications.