Abstract: In ultrasonic testing, the variation of surface roughness of workpiece tested induces various coupling losses, as a result the testing error is inevitable. The common method to compensate the coupling loss is to make various reference blocks according to the material of workpieces, obviously it consumes time and labor. An universal equation, which can be directly used to compute the compensation resulted by the surface roughness, was deduced according to the flat-bottomed echo formula. Reference blocks with various surface roughness were made and used to verify the validity of the equation deduced, and the experimental results proved its correctness.