Challenges such as the “invisibility” of fishway entrances, the “inaccessibility” of chutes, and “post-passage swimming behavior” have long persisted in the restoration of fish migration routes and the comprehension of fish life history. Specifically, urgent issues such as how fish swim after passing through a dam, whether they get entrapped in reservoir “ecological traps”, and if they can traverse the reservoir to reach their habitats require immediate attention. In this study, three typical fish species in the Heishui River of the Jinsha River, namely Schizothorax prenanti, Paracobitis variegatus, and Paracobitis potanini, were chosen as research subjects. The status of these target fish resources was evaluated by means of net-capture methods, and post-passage upstream movement data were gathered using telemetry technology. Subsequently, a quantitative evaluation model of post-passage upstream movement effectiveness was developed, integrating the hydrological and hydrodynamic factors of the reservoir. The key factors influencing this effectiveness were determined, and the response relationships between upstream effectiveness and these factors were established. The results are as follows: 1) S. prenanti, P. variegatus, and P. potanini were dominant species in the area from the upper part of the dam to the fishway and then to the lower part of the dam, accounting for more than 40% of the total catch in aggregate. 2) The fishway passage rate of the three target species was 11.14%, and their success rate of upstream migration from the fishway exit to the reservoir tail was 30.87%. 3) After passing through the fishway, the target fish exhibited different habitat preferences. Seventy percent remained in the near-dam reservoir area, which was the highest proportion; 28% continued to swim upstream to the pebble riverbed area; no target fish were observed to migrate upstream from the gravel braided riverbed area to the meandering gravel-sand riverbed area. 4) The sensitive factors affecting post-passage upstream effectiveness include fish species, flow rate, rate of change in hydrological conditions, and diurnal rhythm. 5) The success rate of upstream migration after passing the dam decreased as the river flow increases; target fish tended to migrate during short periods of rising water; over 60% of target fish chose to migrate at night; the success rate of post-passage upstream migration for S. prenanti was significantly higher than that for P. variegatus and P. potanini.