The invention provides a steel billet solidification detection system based on ultrasonic waves. The system comprises a steel billet and is characterized in that an ultrasonic wave excitation device is arranged on the upper side of the steel billet, and a signal receiving device opposite to the ultrasonic wave excitation device is arranged at the lower end of the steel billet. The true online measuring is achieved, transverse waves excited by electromagnetic ultrasonic are spread at the high temperature, when a liquid core exists, the transverse waves excited by electromagnetic ultrasonic cannot penetrate through the liquid core to reach the other face of a casting blank, the reflection echo time is greatly ahead of schedule, and therefore online measuring is achieved. No matter how the pulling rate, the water distribution amount, the middle ladle temperature, the steel type and the like are changed, the solidification tail end position of the steel billet can be precisely detected; in the process of transverse scanning type detection, the section of the liquid core is detected in a distribution mode; mounting is simple, the whole mounting period is short, and the mounting process does not affect the normal production.