There are two different schools to study the hidden degree of sediment particles: absolute hidden degree and relative hidden degree. The 2-D relative hidden degree of sediment particles defined by Han Qi-wei was elaborated in this paper. The hidden degree formula of wide grain size distribution of non-uniform sediment was deduced statistically. Relevant discussions on the physical significance of the formula were presented, and the concepts of “large grain”, “small grain” and “medium grain”associated with broadly graded nonuniform sediment were proposed as well.