Aiming at the problem of poor positioning and quantification of internal defects in ultrasonic testing of composite insulators caused by high acoustic attenuation, the total focus method (TFM) is used to improve the detection quality. Based on the k-Wave toolbox, the simulation of ultrasonic total focus detection of internal defects of composite insulators is carried out. The effects of frequency, number of phased array elements, defect size and other factors on the detection results are discussed and the detection experiments are carried out. The results show that the full focusing method is effective in detecting the internal defects of composite insulators. The frequency has a great influence on the detection results. When the frequency is larger, the penetration force of the ultrasonic wave is weakened, and the attenuation coefficient of the ultrasonic wave in the composite insulator is increased. When the smaller frequency is selected, the detection resolution of the probe is reduced, which affects the quantification of the defect. Therefore, the appropriate frequency should be selected when detecting the composite insulator. The imaging effect of the 2.5 MHz frequency is the best among the four different frequencies. The number of array elements and defect size have little effect on the detection results. In the actual detection, the appropriate number of array elements should be selected according to the actual engineering needs. The research results provide a theoretical basis for the ultrasonic total focus method to detect the internal defects of composite insulators.