The freight rate index reflects the dynamic relative number of the changing trend and degree of freight rate level in different periods. Usually expressed as a percentage. On the basis of fixing the average freight rate in the base period, the average freight rate in each period (comparison period) is compared with the average freight rate in the base period, and the comparison percentage between the average freight rate in each period and the average freight rate in the base period is obtained. According to the different base periods used in the calculation, it can be divided into fixed base freight index and ring freight index.