Abstract: A complex steel structure system of combining steel frame-supporting system with intersected trusses was adopted as the main structure of Tianjin library． The stress level in key joints was high in construction process due to frequent changes of the main steel structure system，therefore it was necessary to monitor its stress in the whole constructing process． Three methods，electric resistance static strain testing system，electric resistance dynamic strain testing system and vibrating wire strain gauge system were used for monitoring． Results of these 3 ways are in accordance with each other，which guaranteed the smoothness of construction procedure and the accuracy and validity of monitoring results．