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 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 via number theoretic method, the reduced dimension simulation method of track irregularity was developed, by which only two random variables were needed 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, cumulative distribution function of the dynamic system and the dynamic reliability of the guideway beam can be accurately obtain. 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 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. However, since the dominant frequency of the vehicle load is approaching to the basic frequency of the vertical vibration of the guideway beam at the speed of 80 km/h, which makes the value and dispersion of the dynamic factors at this speed the largest. The dynamic factors of the guideway decrease as the weight of vehicle increases. In addition, the track irregularity roughness has a significant impact on the dynamic factors of the guideway beam. If the reliability of the guideway beam is required to be 1, the track irregularity amplitude should be controlled within 5.03 mm.