Abstract: In this paper, the global seismic belts are redivided into five divisions based on the principle of plate interactions. The first belt (1) is the west coastal range of the Americas; the second belt (II) is the northwestern margin of the Pacific ocean; the third belt (III) is the oceanic island arcs between the Indian and Pacific oceans; the fourth belt (IV) is the Eurasia; the fifth belt (V) is the global oceanic ridges. The order of the seismicity level of each belt from high to low is III, II, I, IV, V. The seismicity indexes of the belts I, II, III, for each year of the present century are calculated. The maximum entropy analysis and the periodogram analysis are made for the temporal variations of the A(b) indexes. It is shown that the periodic composition of the variations for the belts II and III is quite different from that for the belt I. The variation of the belt 1 is comparatively smooth and abundant of short-period components with two peaks in 1942 and 1960 respectively. The variation of the belt II had been in a phase of long-term decrease since 1964 and reached the lowest point in 1984. The effect of the 1950s event in the belt III whs prominent. The variation of the belt III can be a good representative of the general trend of the variation of the glowal seismic activity. The variations of the belts II and III have stronger long-jeriod components which may be attributed to the vast area of the Pacific Plate.