With the development of computer technology, digital signal processing has made the quantitative analysis of geological problems become one of heat problems in geological research. Functional functions in Matlab can greatly simplify the procedure of mathematic operation, and build a connection between spectrum analysis of logging curves and Milankovitch cycles among strata. This not only brings great convenient conditions for researchers, but also provides a new thinking for association of mathematic analysis and geology. The method is used to analyze the Permian Changxing Formation in northeastern Sichuan Basin. It is found that the strata have main sedimentary cycles of 13.23~13.37, 4.91~5.35 and 3.60～3.96 eters and secondary sedimentary cycles of 2.18～2.38 and 1.85～1.88 meters. The sedimentary cycles are controlled by palaeoclimate.