To address the low efficiency and incomplete coverage of conventional ultrasonic testing in detecting weld quality at the fillet welds of shield machine cutterheads, a rapid imaging detection method based on ultrasonic phased array is proposed. Numerical simulations were carried out using finite element software to design the sound beam scanning strategy for weld coverage. Simulation results indicate that when a wedge probe with a center frequency of 1 MHz and an element spacing of 0.5 mm is employed, an effective beam deflection ranging from 45°~70° can be achieved, enabling accurate detection of cracks in fillet welds. On this basis, a coherence factor(CF) weighting algorithm was applied to the traditional total focusing method(TFM) imaging to enhance pixel-level consistency, effectively suppressing artifacts and noise, and improving image contrast and edge clarity. The proposed method is specifically tailored for thick-walled cutterhead structures with complex fillet weld geometries. The results provide a theoretical foundation for the subsequent development of dedicated phased array probes and experimental procedures.