Abstract: The analysis and application of mass data is one of critical issues in the field of structural health monitoring for bridges. The analysis methods of mass strain monitoring data and fatigue assessment are presented with the object of the long-term strain monitoring data for the steel-box girders of Runyang bridge. First, the fatigue damage calculating and life prediction methods are established on the basis of S-N curves in Eurocode and Palmgren-Miner rule. Then the fast extraction of stress cycles and elimination of random interference in strain data are investigated. The necessity of long-term fatigue monitoring is discussed for the steel box girder of highway bridges. At last the fatigue life prediction is developed for the welded details of Runyang bridge. The results show that the rain-flow counting program, constructed using a combination of MATLAB and C language, can extract stress cycles from the mass strain data rapidly. The small stress cycles caused by a random interference can be eliminated according to a certain threshold, which is determined on the fatigue damage induced by all levels of the stress cycles. The fatigue damage of welded details fluctuates markedly during a year, thusly a set of short-term data may lead to a considerable deviation of fatigue life prediction.