Nowadays, observing target echo is becoming a very important way of detecting target. Among various kinds of characteristics about target echo to be measured, target strength can be got easily. In this paper, we introduce a model building of detecting the strength of target echo: segment the target surface into a lot of surface elements and then integral about them. With this model, we can calculate target strength on condition of various target shapes and receive distances. Theoretically, scattering sound field depends on directivity of incident sound source, target surface shape, distance between target and sound field, reflecting coefficient and so on. In this paper, in order to focus on surface segmentation and integral, the effect of directivity of sound source will be ignored, while the reflecting coefficient will be assumed to be one. We will consider these two aspects on detecting target strength in subsequent research. It is well known that target strength of rotary and column have lobes in certain direction, meanwhile, the farther between target and receiver, the fiercer target echo we can detecting because of the increase numbers of reflecting elements. As showing in the following experiments, the proposed model can perform the same with transcendental studies. Keywords-model; target strength; echo; segmentation; integral