Through indoor model tests, a lightweight embedded erosion capturer was used for the first time to record dynamically the breaking process of a cohesionless earth dam with broadly graded non-uniform sand-gravel. Under constant upstream incoming flow of 30L/s, the erosion process of the dam (at 1m height) and the hydraulic parameters at the breach in overtopping condition were researched. The whole dam-break process was divided into four phases: in the first phase, erosion notch formed and developed, with the downstream slope erosion proceeded slowly; in the second phase, the water fell in multilevel steps and the downstream slope erosion intensified gradually; in the third phase, the multilevel steps developed into a big waterfall and the dam broke rapidly; in the fourth phase, dam erosion slowed down and tended to be stable. Based on the test results, the failure mechanism was preliminarily analyzed, the erosion process was revealed to be unbalanced, and the hydraulic parameters at the breach were of strong unsteady flow character. This research offers technological support for people to reduce damages caused by dam-break and to apply emergent measures. 