With the decline of conventional crude oil production, the exploitation and application of heavy oil has gradually attracted attention. However, heavy oil with high density, large viscosity and difficult mining is the problem faced by heavy oil production. There are many ways to reduce the viscosity of heavy oil, among which upgrading visbreaking reaction and non-upgrading visbreaking reaction are two important ways to reduce the viscosity of heavy oil. The latter include heating and viscosity reduction, thinning and viscosity reduction, and chemical viscosity reducing and so on. There are two methods for upgrading visbreaking reaction, namely mild thermal cracking and aquathermolysis. The aquathermolysis viscosity reduction technique is a more efficient method of viscosity reduction based on thermal cracking. Under the appropriate reaction conditions and suitable catalyst conditions, the viscosity of heavy crude oil can be greatly reduced, which is a heavy oil mining technology with good application prospects.