In order to provide the scientific basis for improving the temperature sensitivity of singlemode-multimode-singlemode (SMS) based fiber temperature sensor and the t emperature stability of temperature-insensitive preferable sensor based on SMS fiber structure,the thermo-optic effect and thermal expansion effect induced temperature characteri stics of an SMS fiber structure are studied.The theoretical model for analyzing the temperature properties of an SMS fiber structure constructed by normal multimode fiber is established,by analyzing the multimode interference pr inciple and identifying temperature influence factors and their relations to temperature of such a fiber structure.F or the SMS fiber optic structures with different multimode fiber core diameters,the temperature characteristics aff ected by thermo-optic effects of multimode fiber cladding and core,and thermal expansion effects of diameter and length of multimode fiber core are analyzed further by using the theoretical model.The results indicate that t he thermo-optic effects of multimode fiber cladding and core are the first and second contribution to temp erature sensitivity of an SMS structure, respectively.Furthermore,the sensitivities of temperature induced by the above two effects both increase with the decrease of multimode fiber core diameter.However,the two change directions are opposite.The influence of the thermal expansion effect of multimode fiber o n temperature characteristics of an SMS fiber optic structure is very weak and can be neglected.The temperature sensitivity,which is about 12.5pm/℃,induced by all thermal effects remains almost unchanged with different diameters of multimode fiber cor e.