The stainless steel casing structure of large gas turbine engine contains a large number of R angle regions, which are prone to micro-cracks, pores, inclusions and other defects in the manufacturing process because of its large wall thickness and curved surface structure. Due to the shape limitation of the curved surface of the R angle, the radiographic film cannot be arranged, and the R angle area with large thickness is difficult to be penetrated by the ray, so that the sensitivity of the radiographic testing is reduced, and defects are missed. Therefore, the ultrasonic immersion phased array testing for the internal defects in the R corner region of the large stainless steel case is proposed.The phased array probe is arranged on the inner ring curved surface of the casing, and the array element is controlled to transmit ultrasonic waves to form a focused sound field in the R angle area by modifying the curved surface focusing rule. The numerical simulation model is established to analyze the focusing performance of the sound field before and after the modification of the focusing law, and the influence of the water distance on the focusing sound field is also analyzed. Based on the modified focusing rule and the optimized water distance, the ultrasonic phased array testing of the R focal region of the casing is carried out. The results show that a focused sound field can be formed in the R angle region by modifying the curved surface focusing rule and optimizing the water distance, and the image quality of the sector scanning in this region is significantly improved; the transverse hole defect with the equivalent size of Φ 1. 5 mm can be displayed with good resolution, and the quantitative relative error is as low as 6. 7% by use of -6 dB quantitative method.