The reliability model of the main beam of a loading platform was established when the beam strength failure occurs through introducing the improved first order and second moment (AFOSM) technique into reliability analysis and combining the loading characteristics of the loading platform of the rotating stereo garage. Then the limit state carrying capability equations of the beam key sections were built using this new model. The principle which the reliability sensitivity of the main beam changes with the force was obtained through calculating with this model. The influences of the structure parameters on reliability were analyzed finally. These results of the study provide a theoretical basis for the structural reliability design of the main beam.