In order to assess the working condition of concrete box-girder bridges during operation period, an assessment method was proposed combined the structural health monitoring data and the reliability theory. The normal use limit state equation was established by considering the resistance and load effects. In this equation, the concrete tensile strength standard value was taken as the resistance while and the vehicle load response and the vertical temperature gradient load response were taken as the load effects. For comprehensive strain responses acquiring from the bridge health monitoring system, a trend term extraction method based on empirical mode decomposition was proposed. Probability density distribution fitting of the vehicle load responses and the vertical temperature gradient load responses was conducted. Then, the equivalent normalization method was used for the load effect unsatisfying the normal distribution to calculating failure probability under the most unfavorable load combinations. Results show that the monitored vehicle load responses and the vertical temperature gradient load responses are not satisfy normal distribution, and the generalized extreme value distribution could get better fitting result. Moreover, the failure probabilities of the top plate and the bottom plate of the mid-span were less than 1%, and the risk of cracking of bottom plates significantly increases under the coupling of vehicle load and vertical temperature gradient load.