Aiming at the prominent problems including hump characteristics and poor operation stability of nuclear reactor charging pumps under off‑design low‑flow conditions，this paper adopts numerical simulation combined with experimental tests to explore the effects of balance chamber flow passages on the hydraulic external performance and internal flow field of the charging pump，and optimizes the internal flow state by improving impeller structural parameters.Single‑stage model pump experiments and numerical results reveal that the leakage flow rate of the balance drum accounts for merely 3%~6% of the rated design flow.Such leakage exerts a marginal effect on pump head prediction and cannot change the qualitative law of hydraulic hump characteristic prediction；hence，a simplified numerical model ignoring the balance chamber flow passage is applicable for hydraulic performance optimization.Comparative tests on pump hydraulic performance with different blade lean angles （θ=0°，3°，4°，5°） under low‑flow hump working conditions verify that the blade lean design is an efficient method to restrain the hydraulic hump.Specifically，the hydraulic hump phenomenon of the pump is completely eliminated within the full operating flow range when the blade lean angle is set to 5°.Adopting the combined optimization scheme with tapered convergent flow passage and 5° blade lean toward the blade pressure side，the proposed design not only restrains the hump characteristic of the charging pump under low‑flow conditions，but also optimizes the internal flow field of the impeller.Meanwhile，the generation of internal low‑speed flow zones and flow vortices is weakened，which further improves the cavitation resistance and operational stability of the pump.This study can provide technical support and theoretical references for the optimal hydraulic design of nuclear reactor charging pumps.