The viscosity of heavy oil is high, the pipeline transportation is difficult, and the viscosity decreases greatly after dissolved gas. Thus researching viscosity-reducing properties of dispersion system of heavy oil with dissolved gas is of great significance to the actual pipeline operation. The viscosity-reducing mechanisms, viscosity models and influence factors of heavy oil with dissolved gas are summarized. The research indicates that dilution micelle damage, swelling, molecular force and micro-bubbles effect can reduce the viscosity of dispersion system. Viscosity models are mainly established by Shu, Abivin and so on, and the defects in the model are presented. The property of oil and gas, temperature, pressure, the amount of dissolved gas and micro-bubbles have different effects on the system's viscosity, and the direction of the heavy oil with dissolved gas is proposed.