Sufficient down hole steam equality is important for the success of steam injection performance. In this paper, a system model is developed to predict steam quality with variable(T, P) fields in steam injection wells. In particular, the formation temperature, which changes overtime, was examined. A hybrid algorithm for solving the model is used, along with the fourth-order Runge-Kutta method and the finite difference method. Data from a Chinese X Well is used as a case history to verify the calculations and a trend and sensitivity analysis was also done for the model. The effect of the process variables such as injection rate, injection pressure, pipe diameter and heat insulation thickness were put into test for the sensitivity of the model. The results provide technical reliability when designing well tests in deviated wells. (C) 2012 Elsevier Ltd. All rights reserved.