The objective of this study was to explore the combined effect of ultrasound (20 kHz ultrasound probe (12 mm diameter), an output power of 350 W for 5 min (on-time and off-time pulse durations of 2 s and 3 s, respectively)) with low concentration (0.2 M) sodium bicarbonate solution (USB) on chicken breast tenderness and its preliminary molecular mechanism. Therefore, myofibril fragmentation index (MFI), filtering residues, cooking loss, shear force, histology of meat as well as SDS-PAGE, circular dichroism (CD), synchronous fluorescence spectroscopy, differential scanning calorimetrv (DSC) of actomyosin were investigated and compared with untreated (Control), deionized water (DW), ultrasound in deionized water (UDW), 0.2 M sodium bicarbonate solution (SB), respectively. The results showed that USB can effectively increase MFI, and reduce filtering residues, cooking loss and shear force by compared with UDW or SB. Furthermore, myofibril was presented with the largest interfibrillary spaces and the highest degree of actomyosin dissociation in USB group. The increase in α-helix content and decrease in fluorescence intensity of tyrosine and tryptophan implied that USB caused the conformation change in actomyosin. Additionally, actomyosin in USB group became more sensitive to temperature. Therefore, the treatment of ultrasound combined with low concentration of sodium bicarbonate accelerated actomyosin degeneration was considered as a promising and efficient technique in meat processing, especially for the fitness enthusiasts.