Through the summarizing and analysis of the previous researches,we put forward the principles and requirements in designing substrate for extensive green roofs,present a new type of substrate mixture which has higher content of fine particle matter and suits for the regions with the climate of summer drought,and investigate how to design the substrate mixture and check the application effect of our newly-invented additive for the green roofs. We firstly design orthogonal experiments with clay(A),distillers' grains(B),expended clay(C),and cement(D) as the 4 factors(3 levels for each factor),and treat germination rates of Bermuda grass and Ryegrass as dependent variables to find out how the factors and levels influences germination rates of Bermuda grass and Ryegrass by pot culture experiments. Then,we compare the germination rates and growths of Bermuda grass and Ryegrass and growth of Stringy Stonecrop Herb between the most adverse substrate mixture and the substrate mixture added with additive for the green roofs. Results show that:(1) by the significance influencing the germination rates of Bermuda grass and Ryegrass,the orders of the factors are D→A→C→B and D→C→A→B,respectively,and the best substrate mixtures for the two grasses are A2 B3 C1 D3 and A2 B2 C3 D2,respectively. (2) The content of cement in substrate mixture influences significantly the germination rates of Bermuda grass and Ryegrass and the cement-clay ratio should not exceed 18% for the germination of Bermuda grass and 23% for Ryegrass. (3) Stringy Stonecrop Herb indicates good alkali endurance and can survive in substrate mixture with 23% cement-clay ratio. (4) When our newly-invented additive for the green roofs is added into the most adverse substrate mixture of A3 B3 C2 D1 with 23% cement-clay ratio,the pH value of the A3 B3 C2 D1 drops significantly. The pot cultured Bermuda grass and Ryegrass with this improved substrate gain much better germination rate and the pot cultured Stringy Stonecrop Herb grows much higher.