Runner blade crack is a problem that affects the operation security of hydro-turbines. In this study, penetrating cracks were found by inspection on the runner blade of a Francis turbine near hub and shroud on trailing-edge. To solve this problem, prototype test was conducted to find the conditions under strong hydraulic instabilities. The turbine vibration region was classified based on the test and found mainly at small guide vane opening angles or low-head high-load conditions. In these regions, vibration was strong and the numerical-predicted flow regime was disordered. Analyzed by fluid-solid interaction method, stress concentrations were found on the blade-hub and blade-shroud connections on trailing-edge. By excluding the influence of resonance, the runner blade crack problem was found as the combination of concentrating static stress, hydraulic excited pulsating stress and residual stress. Triangle blocks, which were helpful for eliminating the concentrating static stress, were welded on the blades after cutting-off the old crack sites. Polishing and buffing were conducted for reducing potential residual stress. After a long-time re-operation, the improved runner was found without runner blade cracks. Prototype re-testing showed no impacts on the efficiency. This study provided a good solution for the Francis turbine runner blade crack problem under condition-complexity. (C) 2019 Elsevier Ltd. All rights reserved.