Risk evaluation of flowing backward from the main stream in Xiaobei of the Huanghe (Yellow) River to the lower reacher of the Weihe River has become the significant work of flood prevention because of the dangerous situation of the flowing backwark. Based on the contrast of historical observed data and remote sensing images, using the inserting value calculation method, risk degree of flowing backward was evaluated and zoned. The results are as follows: 1) It is obvious that the larger the flood of the Huanghe River, the larger the area of inversely flow deposition, and the more the water level in the reaches of the Weihe River, vice versa. 2) The highest risk regions usually located in the eastness of Dali near by the main stream in Xiaobei of the Huang River and the riverside regions of Tongguan, Huayin, Huaxian, Weinan and Lintong. The higher risk regions are located in the northeastern part of Dali and the common boundary of Weinan and Lintong on the southness shore of mainstream watershed of the Weihe River. Then, the remainder is lower risk regions. 3) Westward moving of the estuary of the Weihe River to the Huang River and flowing backward are the cause-and-effect, which not only resultd in shrinking of the riverbed at the lower reaches of the Weihe River, but also reducing the ability of flood transflux. So there is a complexion of small flux, high water level and big disaster. 4) In order to reduce the risk degree of flowing backward, as the given result, the district regionalism and local protection must be broken to implement the integrated program of the watershed and to decline the Tongguan stage to stabilize the physiognomy at the estuary of the Weihe River to the Huang River.