This study was to investigate the germicidal efficacy and mechanism of ultrasound combined with lactic acid against Salmonella planktonic and biofilm cells. The results showed that ultrasound (400 W) combined with lactic acid (0.5% and 1.0%) had synergistic germicidal effects. The ultrasound (400 W) combined with 1% lactic acid could inactivate Salmonella planktonic cells to zero within 30 min， and could inactivate biofilm cells to lower than the detection level within 60 min. Scanning electron microscope(SEM)， confocal laser scanning microscopy， and the ATP leakage analysis showed that ultrasound combined with lactic acid could seriously damage cell morphology， increase the permeability of cell membrane and cause the release of intracellular ATP. In addition， the activity of respiratory chain dehydrogenase significantly decreased after treatment with ultrasound combined with lactic acid. Therefore， ultrasound combined with lactic acid had synergistic effect against Salmonella planktonic and biofilm cells.