It is very helpful for power system operation to assess and forecast the uncertainty of the load forecasting. The improved credibility assessment index of the short-term load forecasting results is presented to assess the uncertainty in this study. The forecasting method for credibility of short-term load forecasting is also proposed by adopting genetic algorithm-based back propagation neural network. In the case study, it is proved that the improved credibility assessment can evaluate the short-term load forecasting results and the forecasting methods. Meanwhile, the credibility forecasting can contribute to optimise reserve capacity in power system scheduling.