A method for studying fatigue reliability variation of urban rail vehicle frame based on measured stress data was put forward． In order to establish the relationship between structural damage and reliability，firstly，a new heuristic algorithm was used to fit the probability density function of the stress spectrum for each day． And a probabilistic cumulative damage model considering load interaction was derived based on the statistical distribution of stress． Then，considering the difference of the results of different sensors in a certain structure area，the gene- ral variation law of structural reliability with operational mileage was obtained by using Wiener process with random effect based on Probabilistic Damage statistics and Markov chain-Monte Carlo method． The calculation results show that the reliability calculation method can obtain the fatigue reliability results of the frame which are consistent with the engineering experience．