In order to improve the wave attenuation performance of a box-type free-surface breakwater,the bottom contour of the box is transformed from flat to symmetrical wavy by modifying the bottom contour of the box,so a kind of symmetrical wavy-bottom free-surface breakwater is proposed.The physical model tests are carried out for flat-bottom and symmetrical wavy-bottom respectively.During the tests procedures,the main bodies of the two breakwater models are fixed in the small wavemaker flume through the brackets,and the flume water depth are kept unchanged.A series of experiments are carried out through changing the box immersion depth,the wave steepness and the wavelength.The variation of transmission coefficient,reflection coefficient and energy dissipation coefficient with relative width B/L is given.By the performance comparison,it is found that the wave attenuation performance of symmetrical wavy bottom is better than that of flat bottom,and the increased quantity of wave attenuation effect is up to 60%;the reflection coefficients of the two models are almost the same,which indicates that changing bottom contour of the box has little effect on the reflection performance of the breakwater;the symmetrical wavy bottom dissipates more wave energy than the flat bottom.