Affected by the randomness of track irregularity, the dynamic factor of the guideway beam caused by the maglev vehicle has obvious uncertainty. In order to accurately calculate the reliability of the dynamic factors of the guideway beam, a vertical coupling vibration model of the medium and low speed maglev train-bridge system was established, which was composed of the maglev vehicle, the simply supported girder and the suspension control system. Then, combined with the idea of orthogonal random function and the strategy of selecting points by the number theory method, the reduced-dimension simulation method of track irregularity was developed, which repaired only two random variables to simulate the representative samples of the track irregularity. Finally, based on the probability density evolution method and the equivalent extreme-value principle, a reliability evaluation method for the guideway beam was proposed, by which the probability density function, the cumulative distribution function of the dynamic system and the dynamic reliability of the guideway beam can be accurately obtained. The Changsha low-medium speed maglev line, as a case study, was numerically analyzed. The reliability of the calculation model was verified by comparing with the results of field measurement and Monte Carlo method. Furthermore, the effects of vehicle speed, vehicle weight and track irregularity roughness on the dynamic factors and the reliability of the guideway beam were discussed. The results indicate that the reliability of the guideway beam is 1 within the speed range of 60 to 140 km/h. The weight of vehicle has a small effect on the dynamic coefficient of the guideway beam, but the effect of the track irregularity roughness is significant. Overall, the Changsha low and medium speed maglev 25 m simply supported girder bridge has good dynamic performance and a large safety margin.