Magnesium Alloy Cartridge Case is a combined revolution body with the feature of variable thickness and curvature. According to its structural characteristics, a scheme of automatic ultrasonic testing was presented, and a corresponding automatic ultrasonic testing system was designed. The kinematics model of 6-DOF testing platform was built to realize the accurate control of the ultrasonic scanning motion path and probe position. An ultrasonic measurement model for case defect was proposed and the defect size characterizing curves were drawn on the basis of defect scattering models and multi-Gaussian ultrasonic beam models inside the case. An ultrasonic scan experiment was conducted by using 6-DOF testing platform, and the validation of the work was verified by the realization of the detection and sizing of artificial defects.