Abstract: In view of the protection and renovation of the Great Wall in Beijing, the principle of "unchangeable status" in the Regulations on the Protection of the Great Wall should be followed. As the main building material of the Great Wall in Beijing, the understanding of strength performance of the grey brick by testing is particularly important. It has important reference significance for the structural safety of the Great Wall, no matter whether old bricks are to be replaced or not, or the selection of strength for new bricks. The old grey brick samples of Ming Dynasty Great Wall from six counties in Beijing are selected. The acoustic time of the brick samples is measured by ultrasonic detector, and the ultrasonic wave velocity is calculated. According to the actual compressive strength of the grey bricks obtained from the test, the relationship curve between the wave velocity and compressive strength is established. That is to say, the compressive strength of the grey bricks of original Ming Dynasty Great Wall in Beijing is deduced from the ultrasonic nondestructive testing technology.