Abstract: The double-matrix array（DMA）probe was used for the ultrasonic testing of non-fusion of groove in the GH3535 alloy weld of the Chinese molten salt reactor. Due to the anisotropy and inhomogeneity of the GH3535 alloy weld, the sound beamed propagation direction deviates and decays, which affect the accuracy of the positioning and quantification of the defects. Combined with the process parameters such as focusing sound range path and probe frontier position, which affected signal-to-noise ratio, defect quantitation and position, the process for defect detection was analysed and the effective process parameter was proposed. The results showed that when the probe frontier position was set to −6~6 mm and the focusing sound path was set to 13~28 mm, non-fusion of groove at a depth of 12.9 mm could be effectively detected and the signal-to-noise ratio was more than 12 dB. When the probe frontier position was −6 mm, the sound beam could cover the whole weld, and the focusing sound path was 13~28 mm, the defect equivalent value was ϕ 2.5 mm +1 dB~−1 dB, the defect positioning deviation was less than 1.4 mm.