Abstract: Hydropower development provides substantial benefits to human society but simultaneously disrupts river connectivity. As the primary facility for fish passage, fishways often neglect illuminance as a key environmental factor in their design and operation, resulting in reduced passage efficiency. To analyse the effects of illuminance on the sustained swimming capacity and upstream swimming performance of fish and to improve fish passage efficiency in fishways, five illuminance levels (0, 10, 100, 1 000 and 10 000 lx) and five flow velocity levels (0.25, 0.35, 0.45, 0.55 and 0.65 m/s) were established in this study. A stepwise velocity increment method was used to determine the sustained swimming capacity, upstream holding velocity, and upstream swimming intensity of juvenile Ctenopharyngodon idella and Hypophthalmichthys molitrix. A comprehensive analysis under all illuminance conditions showed that: ① no significant difference in sustained swimming capacity was observed between the two species when the flow velocity was lower than 0.65 m/s; however, when the flow velocity exceeded 0.65 m/s, the sustained swimming capacity of juvenile C. idella was significantly higher than that of juvenile H. molitrix; ② juvenile C. idella exhibited significantly improved swimming performance at 100~1 000 lx, whereas juvenile H. molitrix showed a significant improvement at 10-100 lx. Within these illuminance ranges, the visual orientation ability of both species was fully activated, resulting in a significant enhancement of swimming capacity. The interaction between illuminance and flow velocity significantly affected the swimming behaviour of both species. However, comparison with previous phototaxis studies suggests that the swimming responses observed under combined illuminance–flow conditions are not directly related to phototactic behaviour. In addition, the two species adopted different swimming strategies within their optimal illuminance ranges: juvenile C. idella tended to maintain upstream movement in high-velocity zones, whereas juvenile H. molitrix preferred low-velocity zones.This study provides quantitative data for the design of fish passage facilities and contributes to the optimisation of environmental conditions in fishways to improve fish passage efficiency.