It is the most important in Floristic Geography to determine floristic region,geographical features of transitional floristic region and evolution of flora by correcting and using the coefficient of similarity.The author has commented the formulas and definitions of different coefficient in floristic research,gives the definition of the coefficient of similarity of Flora,introduces exit problems and solved the way in application of coefficient of similarity to floristic geography in the Paper.It is suggested that adopting a same and scientific mathematical formula S C=2c/(A+B)×100% in the comparison study of floristic Geography in the future.