The thermal characteristics of ball screws in servo systems were analyzed,then a simplificated temperature filed model was established to predict the temperature distribution and variations of the ball screws quickly and accurately. The simplified model can predict the unsteady temperature rising of the ball screws, however, there are big errors when the model was used to predict the steady temperature variations of the ball screws. A new simplified model was built by introducing the factor of time modification in the simplified model. The new model has a good performance in predicting the process of steady temperature, however, there exist big errors when the new mode was used to predict the unsteady temperature rising of the ball screws. Based on the performance of the two models abore, a new interval model was also built and the parameters of the new interval model were identified by the LS method. The results show that the error of temperature prediction is less than 0.5 ℃ for a single thermal source, and less than 0.8 ℃ for the double thermal sources.It concludes that the interval model built in the paper has good performance in prediction the temperature variations of ball screws.