In order to clarify the influence of the spontaneous radiation of different-shape shields on detectors and find out the shape that has the least impact, the spontaneous power to the detector from the inner shields with different shapes was systematically analyzed by theoretical calculation. The shapes of the shields include paraboloid, hyperboloid, semi-ellipsoid (including hemisphere), and frustum of cone (including cylinder and cone). The results showed that the spontaneous power to the detector of all shapes of shields first decreases and then increase with the increase of the ratio of the height of the shield to the radius of the bottom. The maximum radiation power is a constant related to the size of the detector, and the minima appear when the heights of the shields are close to or equal to the radiuses of the bottoms. When the shield is semi-ellipsoidal and the height is equal to the radius (i.e., hemisphere), the power of spontaneous radiation to the detector is less than that of any other shield, and it has nothing to do with the size of the shield or whether there are small holes on the top. This study can provide a valuable reference for the design of radiation shields.