Fractal theory was used to study the rules of Yellow River’s food happening. The author put forward a method, grid method, to calculate the riverway’s fractal dimension. After comparing the fractal dimensions and flood peak discharges of the Lower Yellow River parts, some helpful conclusions have been achieved. At a certain scale range, riverway of the Lower Yellow River shows some fractal characteristics, that is self-similarity. The fractal dimension of the Lower Yellow River changes with the flood peak discharge. Usually, the bigger the fractal dimension, the more curving the river way, the bigger the flood peak discharge, the bigger the possibility that flood happens. As a result, the fractal dimension can be regarded as a quantitative index to predict flood.