Method for calculating temperature, pressure and dryness of injected steam and nitrogen mixture

The invention provides a method for calculating temperature, pressure and dryness of an injected steam and nitrogen mixture. The method comprises the steps as follows: calculating initial temperature, initial gaseous dryness and initial steam dryness of a mixture after steam is mixed with nitrogen; calculating wellhead temperature, wellhead pressure and wellhead dryness distribution of a surface pipeline wellhead with randomly set step length by using the outlet of a steam boiler as an initial calculating point; calculating wellbore temperature, wellbore pressure and wellbore dryness distribution with randomly set step length by using the wellhead as an initial calculating point; and providing guidance for the implementation of the corresponding technology in an oil field site based on the wellbore temperature, the wellbore pressure and the wellbore dryness distribution. The method of the invention has excellent computational stability and higher computational accuracy, and provides reliable calculation means for optimal design of wellbore injecting parameters of a stream and nitrogen mixed system.