In the continuous gas lift process, the produced liquid flow rate is a function of gas injection rate and injection depth. In general gas lift design procedures, different physics of temperature variation in annulus and tubing such as heat transfer between the triple systems of liquid slug, injected gas and formation as well as Joule-Thomson effect are ignored. In this paper a new method is developed to obtain the temperature profile in annulus and tubing in gas lift design. This model considers all the real conditions such as heat transfer in mentioned system, joule-Thomson effect and potential changes in both conduits. Using the developed model enable us to calculate changes in density of injected gas and calculate the depth of injection more accurately. The developed model has been applied on wells in Iran's Aghajary oil field. Results show how ignoring the heat transfer and Joule-Thomson effect causes substantial error in gas lift design. From our experience and according to results of this simulator, Generally it can be said that calculated injection depth from classic method and developed model has a difference between 200 and 400m for aghajary field wells where the total depth varies from 3000 to 4200m.