An improved multi-sensor data fusion algorithm based on Dempster-Shafer evidence theory and a criterion of the object evaluation are presented in this paper. The algorithm and the criterion investigate the monitor data of the indoor air that is the research object, and evaluate the quality grade of the indoor air. Experiment shows that the improved multisensor data fusion algorithm based on the Dempster-Shafer reasoning and the resulting evaluations are correct and solve the uncertainty problem which is produced in the multi-sensor information process.