With the completion of 3GPP LTE standard，the commercial application of LTE can be expected soon．The paper gives a Functional Model of the RRC sub layer in LTE system and pays more attention to the assignment of the RRC sub-layer and the state transitions between them．There is a detailed description of the causes and processes for RRC connection reestablishment in this paper．In addition，it also includes the study of RRC Connection Reestablishment．Researches show that it plays an important role for reconfigure SRB1 and resumes data transfer only for this RB．