A method for forming an image sensor device is provided. The method includes providing a semiconductor substrate including a front surface, a back surface opposite to the front surface, at least one light-sensing region close to the front surface, and a first trench surrounding the light-sensing region. The method includes forming an insulating layer over the back surface and in the first trench. A void is formed in the insulating layer in the first trench, and the void is closed. The method includes removing the insulating layer over the void to open up the void. The opened void forms a second trench partially in the first trench. The method includes filling a reflective structure in the second trench. The reflective structure has a light reflectivity ranging from about 70% to about 100%.