Abstract: This paper presents a new high-precision method for calculating sedimentary cycles. The grayscale values acquired from the sample section by digital image analysis software can be used as a substitute index of lamination rhythmic changes. Through least squares and first difference data processing, the sedimentary cycle rhythm can be quantitatively estimated. The algal dolomite laminae collected from Middle Triassic Leikoupo Formation in southwest Sichuan were studied. The obvious tidal sedimentary rhythm laminae displayed on polished sections were observed under microscope. The application of this method to the sample led to the discovery of 217 tidal cycles, with the maximum thickness being 4.2 mm. A comparison based on manual counting confirmed the feasibility of this method. Compared with traditional methods, this method is more time-saving, more efficient, and more easily extended. It can also be applied to other types of sedimentary cycles.