Ultrasound is relative to sound waves, the human ear can hear the frequency range of sound waves is 20 ~ 20,000Hz (Hz), less than 20Hz sound waves called infrasound, more than 20,000Hz sound waves called ultrasound. Ultrasonic waves due to the good directionality, when through the interface of different substances will be reflected, the reflection of ultrasound and the density of the material through the physical properties, etc., with a specific receiver to receive back the reflected ultrasound, through the characteristics of the reflected wave can understand the physical properties of the material through which ultrasound. If computerized, these reflected waves can be converted into images and used to diagnose diseases. Ultrasound is the preferred means of examination for liver cancer. Proper ultrasound examination can detect liver cancer with a diameter of 2 cm and experienced physicians can diagnose liver cancer with a diameter of 1 cm; based on the characteristics of ultrasound, liver cancer can also be distinguished from other occupying lesions, such as liver cancer showing substantial hypoechoic areas and substantial inhomogeneous echogenicity, while hemangioma shows hyperechoic areas and liver cysts show non-echoic areas. Ultrasound has the characteristics of sensitivity and accuracy, convenient examination and economic cost, which makes it play an important role in the diagnosis of hepatocellular carcinoma. Due to the popularization of ultrasound in urban and rural areas of China, the early diagnosis level of liver cancer has been greatly improved.