Multi-mooring system is one of the popular means to position a floating offshore structure on the ocean. On doing basic design of it, we have to make static and dynamic analyses of mooring lines, but of the two the static one is more basic. Till now there were no useful method to make statical analysis considering sea bed condition, and so we were in trouble with making mooring design of floating structure in the ocean region that have complicated sea bed condition. Authors tried to express sea bed condition by assuming the sea bed to be made of small straight lines and made statical analysis by using difference equation method. The numerical results were in very good agreement not only with experimental results but also with catenary theory. Further we made design charts using parameters such as friction between line and sea bed, line scope, slope and roughness of sea bed, weight of sinker, buoyancy of float, line's extension, and analyzed the effects of these parameters have on mooring characteristics.