Abstract: 2 artificial defects in a thick-walled scorched steel plate were detected and imaged with focused ultrasonic shear horizontal(SH) waves. An inspection and imaging system with 20 channels of ultrasonic shear horizontal waves was developed on the purpose of detecting the thirteen defects, being spaced out 30±1 mm apart and ranged vertically within 10~380 mm deep, with the dimensions less than one detection wavelength(3.09 mm). On the theory of synthetic aperture focusing for long distance inspections, the signals from the related channels were processed and imaged. It was achieved that the defects were accurately located, keeping the error range of dimensions no more than one detection wavelength. The method provides reference to the in-line detection of large scale steel equipments in high-temperature and pressure environment.