Abstract: To address the problems of difficulty in identifying the depositional period from seismic data and spatial morphology of riverine sand body in the giant thick sand geological body in Member 3 of Huagang Formation (H3 in short) in Y Structure, western subsag, the Xihu Sag, was selected as the research object. Based on the sedimentary characteristics revealed in boreholes, the main frequency for implementing harmonic frequency enhancement technology was determined via sand body combination forward simulation, and isotime stratigraphic framework were established to identify sedimentary cycles by combining the generalized S-transform and wavelet transform. the H3 of the study area could be subdivided into three short-term cycles, namely, H₃⁴, H₃³, and H₃²-H₃¹. The root-mean-square amplitude (RMSA), coherent energy (CEA), and instantaneous phase (IP) attributes were selected via correlation analysis between the attributes and sand thickness. The internal structure and planar distribution characteristics of branching channels were depicted by using multi-attribute fusion, in which the geological information and capability of the sensitive attributes were combined. Constrained by the interpretation of seismic facies within a typical section sequence framework, the internal structure and planar distribution characteristics of branch channels were characterized. Results show that the H3 section of the Y Structure is characteristic of a typical shallow-water braided river deltaic plain, with a large thickness of sand body, and obvious multi-phase superposition of normal grading sequences in vertical direction, which reflects the characteristics of ‘sand-wrapped mud’. The development of three branch river channels in NE-SW orientation, and the distribution of sedimentary micro-phase was controlled by the relative height of the lake level. During the period of H₃⁴-H₃¹, the lake level kept rising, river channels were intertwined in large area, and the largest lake flood surface was formed during H₃¹, during which river channel diverted and the size of river channel continued to shrink. Seismic geomorphological mapping method that integrates frequency enhancement, seismic phase, and multi-attribute fusion provided a new approach for effective deciphering of thick sand bodies in shallow water braided river delta plains.