The steam quality on the top of oil well is one of the important parameters during the thermal oil, and calculation software for the steam quality was developed by VB6.0,and the accuracy of the software was validated by experimental measurement data calculation. The impact the structural parameters (pipe diameter、the thickness of thermal insulation layer)and operating parameters(steam temperature、steam pressure and pipe inlet steam flow) of the steam quality and its change rule were analyzed systematically. Research results show that the steam quality on the top of oil well increased with the increment of the thickness of thermal insulation layer and pipe inlet steam flow and decreased with the increased of the pipe diameter、steam temperature and steam pressure. The thickness of thermal insulation layer was of the greatest impact among such factors, when the thickness of thermal insulation layer was greater than 60 mm and less than 80 mm, and each time the thickness of thermal insulation layer increased 20 mm, the steam quality increased 0120%.