Members of the GH family demonstrate a modular architecture composed of one or two catalytic modules connected to several kinds of accessory modules. In order to further engineer enzymes for industrial hydrolysis of cellulose, it is important to understand the structure and functional relationship of cellulases and the accessory domains. The non-processive endoglucanase Aa_Cel9A from thermoacidophilic bacterium Alicyclobacillus acidocaldarius belongs to the subfamily E1 of family 9 of glycoside hydrolases, members of which have an N-terminal immunoglobulin (Ig)-like domain followed by the catalytic domain (CD). The function of  Ig-like module has not been determined but its presence is required for activity. Deletion of the Ig-domain promotes complete loss of enzymatic activity in a related cellobiohydrolase, CbhA from Clostridium thermocellum. While there is kinetic and structural information of the Aa_Cel9A, molecular dynamic simulations (MD) provide a method to piece together the activity, kinetic, biophysical and structural information to offer insights into domain motion, domain interactions and rate-limiting conformation to help piece together the dynamic view of enzymatic hydrolysis of cellulose. The present work takes a special case with newly resolved crystal structure of endoglucanase Cel9A from thermoacidophilic alicyclobacillus acidocaldarius (CelA) and explores the function of N-terminal Ig domain by simulation approaches. Molecular dynamics simulations (MD) combined with simplified model were performed on the structures of CelA with and without Ig domain. Umbrella Sampling /free energy perturbation (UM/FEP) are also performed to obtain unfolding free energy landscapes for both cases. Both methods show that Ig domain stabilized the structures of catalytic domain.