In the design stage of Cables in Distribution Network, the maximum allowable current carrying capacity of cables is usually designed in consideration of extreme weather, resulting in low utilization rate of the whole cable in most cases. Taking the three-core cable in the distribution network as an example, in this paper, Firstly, based on Monte Carlo method, probability statistics of relevant factors affecting cable ampacity and historical load are carried out. Secondly, the calculation process of the most reasonable allowable cable load based on probability method is designed. Finally, the cable allowable load is obtained by counting the uncertain parameters such as soil thermal resistance, environmental temperature, historical load in a certain period of time in a certain area of Shanghai. The results show that the cable allowable load exceeds 347A specified for the worst environment in 94% of the time period, which verifies the effectiveness of the calculation method.