Abstract: The computation and forecast models (3-5-1), which include 3 neurons in input layer, 5 neurons in hidden layer and 1 neuron in output layer, are set up for concrete carbonation depth, based on optimized BP algorithm. In the computation model the water cement ratio, unit cement consumption and sand percentage are taken as inputs with concrete carbonation depth as output. But, in the forecast model the water cement ratio, unit cement consumption and tine for concrete exposure are taken as inputs with the carbonation depths of those concrete constructions/members which have been exposed to the weather for 20 years and 30 years as outputs. The computation results conform with the test results very well, and the forecast results are favorable.