The lifetime prediction of concrete under field exposure was established by a general method for the remaining useful life of concrete with multiple degradation factors. The mass loss and ultrasonic velocity were selected as the key degradation factors. Considering that the different degradation factors were characterized by their different degradation paths, marginal distribution functions of the remaining useful life of concrete were obtained based on the measured data. After that, Copula function was adopted to establish the relationship between the degradation factors. The marginal distribution was mixed together, and the remaining useful life joint distribution function of concrete was obtained. The results indicate that selection of mass loss and ultrasonic velocity will be available to establish the degradation model of concrete in field exposure environment, and the relevant marginal distribution function of the remaining useful life will be obtained. The service life of concrete under field exposure in Xining is about 225 months, and the damage development period begins at about the 180th month.