Abstract: Composite exchange rate index can be obtained through the ratio of average exchange rates of tow periods, or weighted average of single exchange rate index. These two methods will reach unanimity while the former exclude the influence of trading structure movement to get the fixed composite exchange rate index, and the latter weighted by “mixed currency” of reporting period calculated with the exchange rate of base period. Being regarded as a kind of price, the compilation of exchange rate index is similar to common price index. It is featured by the weights of “mixed currency” of trading volume according to the requirements of itself as a relative. And the average index is merely a variety of composite exchange rate index rather than an independent form.