The effect of tube juncture on heat loss in viscous oil extraction is numerically investigated. On the basis of calculation, a method is presented that proposes how to use a simple configuration of tube juncture to replace the complex one in the process of heat loss calculation in viscous oil extraction, and is called as 1/3 equivalence method. The method has been applied to the practical steam inject tube's heat loss calculation in viscous oil extraction, and the result shows its feasibility in practical application.