A sky brightness photometer with a side-baffled structure, which adopts multibaffles to suppress diffracted stray light layer-by-layer in the inner field of view, is proposed. In addition, specially designed upper baffles block both scattered and diffracted stray light in the outer field of view. A mathematical model is established to simulate the baffles, and calculated results show that the field of view of the photometer can be designed to be 3.5~10 solar radii by optimizing the geometric parameters of the baffles. The stray light in the full field of view can be suppressed effectively to less than 10-8 of the mean solar brightness. Compared with the High Altitude Observatory, which has a sky brightness monitor with a field of view of 4~8 solar radii and a stray light level of 10-7 of the mean solar brightness, the proposed photometer expands the observable field of view and improves the stray light suppression level by over an order of magnitude.