On the basis of elastic theory, a seven parameters model is built. This model is used for judging the ultimate loss of? efficiency in a symmetrical underground tunnel. Through this model, the range and strength of temporary supports in rheological rock can be determined reasonablly. The model result shows that: the supporting range has a positive correlation to the stress of surrounding rock, and presents a negative correlation to the strength of the supported object; with the specified composition of stress state and supporting strength, no matter where the reinforced range is widespread, the temporary support can not work effectively. Finally, the calculating method of parameters in this model is discussed, and the example of how this model is applied is given.