Abstract: The authors previously introduced a parameter A(b) to describe the level of seismic activity. It is named seismicity index. In this paper, A(b) is used to show the temporal variation of global seismicity by assigning an index number to each year up to the beginning of the 20th century. The magnitude data in catalogues of global earthquakes prepared by Duda and Abe were used to calculate the seismicity index. It has been found that the seismicity level derived from Duda's data is much higher than that derived from Abe's, especially for the time interval 1964-1977. The authors thought that it would be better to choose the Abe's catalogue as the principal basis to conduct the calculation.The secular variation of A(b) in the 20th century is a slow decrease with short period fluctuations. Maybe the variation belongs to a descending branch of a long-term periodic variation, which has a period of hundreds of years. The most dominant feature of A(b) of global shallow earthquakes is that there was a sudden drop of A(b) in the time interval of 1952-1955, and this kind of sudden drop of A(b) for deep and intermediate earthquakes happened in 1950-1952. The variations of shallow and deep (including intermediate) seismicity are nearly synchronous before the 1950-1955 sudden drops, and after that time interval the variation tendencies appear to be opposite and complementary to each other. This phenomenon may probably lead to the supposition that the earthquake potential energy storage controls the seismicity pattern from the earth's interior.