Wave transmission, reflection and energy dissipation characteristics of a pile-supported Free Surface H-type Breakwater (FSHB) were experimentally studied under the action of random waves. Large scale laboratory experiments were conducted using a wave flume. The test models of different drafts were subjected to random waves of varying characteristics in different water depths. The experimental results revealed that the fixed FSHB attenuated the incident waves mainly by wave reflection. The wave suppression ability of the fixed FSHB was significantly enhanced when the breakwater was confronted by steeper waves. Most of the wave energy dissipation was associated with wave trapping between arms of the structure during wave overtopping and the eddies generated under keel of the structure during wave-structure interaction. Furthermore, the results indicated that increase in immersion depth of the structure increased its wave attenuation ability. © 2016 Taylor & Francis Group, London.