Ultrasonic diffraction time difference method was applied to 304/Q345R stainless steel composite plate atmospheric storage tank. Firstly,the surface blind zone with butt weld was measured by direct wave method. Secondly,the welding simulation test block of composite plate was tested,and the difference of detection effect based on cladding side and base side was analyzed and compared. Finally,the developed detection process was applied to a 25 000 m,3, stainless steel composite plate atmospheric pressure storage tank,and one buried crack was detected and verified by anatomy. The results show that the TOFD detection technology based on cladding side can be applied to open tank inspection of composite plate atmospheric tank.