The influence index system of process characteristics to buffer size was built from the following 4 aspects: the process risk preference,the execution time uncertainty,the complexity and the starting flexibility.The unascertained measurement theory was introduced to calculate the comprehensive measure vector of process characteristics to buffer size influence.Then the buffer size was qualitatively decide according to the moderate influence of process characteristics and quantitative calculated by confidence identifying criterion.Examples show that the influence factors of buffer fall in [0.177,0.804] from process A to Z,and the influence factors consumed will be rationally readjusted with process characteristics,which verifys the validity of this approach.