Fully utilized the detection signal, phased array ultrasonic total focus method (TFM) imaging can achieve high imaging precision and identify defects, which is one of the most promising future phased array imaging algorithms. However, the TFM imaging can not achieve high resolution imaging of defects, and can not make accurate qualitative and quantitative analysis. Therefore, in this paper, the finite element method (FEM) is used to simulate the full matrix capture (FMC) process of phased array sensors. A TFM imaging program is designed based on the full matrix data. TFM imaging is performed on two typical defects of circular hole and crack, and the TFM imaging law of typical defects is studied. The factors affecting phased array ultrasound imaging are analyzed from the perspective of the defect scattering. The results show that phased array ultrasonic sensor can only receive the partial scattering information of the defect in the actual detection. Moreover, the scattered field distribution of phased array ultrasound is related to the type, size, angle of the defect and incident wave type, and incident angle and so on. Thus, the main energy of the defect scattering can be received is the key factor to the accuracy of the TFM imaging.