Abstract: In order to ensure the safety performance of liquid rocket engine injector, a quantitative detection method of weld penetration depth was carried out based on ultrasonic phased array relative arrival time technique (RATT) for the electron beam welding line with complex structure. A water immersed RATT detection method was proposed firstly. And then, an experiment system was built, taking the CIVA as simulation software to carry out the detection simulation. According to the section structure of the injector, two kinds of detection samples were designed and manufactured to do correlative detection experiments with the samples. Comparing the ultrasonic detection results of the welded samples with their metallographic testing results, the effectiveness of the proposed detection method was verified. The results show that the water immersed RATT can overcome the detection difficulties of injector, including small size of detection surface and the existence of grooves. Realizing quantitative detection of the penetration depth for electron beam welding line of injector, the detection accuracy can be better than 0.15 mm.