Abstract: To achieve the detection and evaluation of corrosion on the internal valve stem of in-service wellhead valves, a phased array ultrasonic testing method with end excitation was proposed to address key issues such as difficulty in disassembling and testing the wellhead valve stem, and difficulty in directly penetrating the valve body for internal testing using non-destructive testing methods such as magnetic powder, eddy current, and ultrasound. Firstly, a simulation model was established using COMSOL finite element analysis software. The sound field distribution pattern of typical valve stem structures was studied and the echo characteristics of the structure was analyzed. Then, valve stem specimens with defect depths of 2-8 mm were manufactured, the valve stem specimens with artificial defects were tested and installed inside the valve for testing, the positioning of the valve stem characteristic structure and defects was performed. The simulation and experimental results indicated that phased array ultrasonic excitation at the end of the valve stem can effectively identify defects with a depth of 2 mm inside the valve, verifying the feasibility of phased array ultrasonic testing without dismantling the valve stem.