An improved modal decomposition method applicable to the analysis and reconstruction of the measured flow fields of internal solitary waves includes the following steps: S1. generating internal solitary waves and measuring a two-dimensional flow field sequence during the interaction of the internal solitary waves with the terrain; S2. extracting a target information from the two-dimensional flow field sequence and obtaining a flow field snapshot; S3. stitching the snapshots into a snapshot matrix in time order; S4. plotting the information density curve; S5. normalizing the information density curve and determining split points; S6. setting the snapshot matrix between two adjacent split points as a linear evolutionary characteristic stage; and S7. performing dynamic mode decomposition on the linear evolutionary characteristic stages.