A laser-ultrasonic testing system was built for the purpose of liquid core detection of a moving and inaccessible continuous casting slab. Reflected waves from different solid-liquid interfaces are studied using the simulation model established with the finite element method. The solid-liquid interfaces of a sandwich structure aluminum plate were preliminarily detected and reconstructed using the time of flight data of ultrasound propagated between layers. Multiple equal-interval ultrasonic echoes were detected and analyzed according to the reflection and transmission of elastic waves in the solid-liquid interface. A B-scan map for the sandwich structure sample was obtained to show the difference between echoes reflected from each interface. Interfaces of different samples were used to calculate the interface position with relative error no more than 1.3%.