The temporal and spatial variations of the void fraction under a breaking solitary wave on a 1:20 sloping beach was investigated through laboratory measurement. The fiber optic reflectometer (FOR) was applied to measure the instantaneous void fraction inside the bubble plume in the experiment. A high-speed camera was used to capture the whole process of the bubbly flow passing through the region where the FOR instrumented. The void fraction distributions in the splash up and cavity areas were well captured by the FOR technique in the experiment. The measured maximum ensemble averaged void fraction occurred in the collapsing cavity area with the maximum value of 65% after wave broke. The ensemble averaged void fraction was more than 20% in the fully mixed region in the surf zone and sustained for a long duration of time. The time series of the measured ensemble averaged void fraction had good synchronization with the instantaneous images taken by high speed camera, so that the correlation between fluid density and velocity variations can be further investigated.