The objective function of minimum cost is established for double layer torsional reticulated shell, taking the rise-span ratio, grid size and shell thickness as design variables. Based on the exhaustion and 0.618 methods, the computing program LSD has been developed. For the double layer torsional reticulated shell with the span of 50 m to 80m, the optimal grid size and thickness have been obtained for systems of light roof materials with different spans. Empirical formulas are deduced from the results.