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We demonstrate a simple but efficient way to optimize and improve the properties of laser-wakefield-accelerated electron beams (<i>e</i> beams) based on a controllable shock-induced density down-ramp injection that is achieved with an inserted tunable shock wave. The <i>e</i> beams are tunable from 400 to 800 MeV with charge ranges from 5 to 180 pC. <i>e</i> beams with high reproducibility (of ~95% in consecutive 100 shots) were produced in elaborate experiments with an average root- mean-square energy spread of 0.9% and an average divergence of 0.3 mrad. Three-dimensional particle-in-cell (PIC) simulations were also performed to accordingly verify and uncover the process of the injection and the acceleration. These tunable <i>e</i> beams will facilitate practical applications for advanced accelerator beam sources.