The present paper is aimed to introduce the authors' idea for the application of the five-element connection number to the quality assessment of the surface water environment.The so-called five-element connection number is referred to as μ=a+bi+cj+dk+el,which is by nature the extended use of the element connection number,such as similarity,difference and counter μ=a+bi+cj of the theory of set pair analysis.On the basis of the four-element connection number,according to the quantitative relation of a,b,c,d,e...