The invention relates to a method and system for calculating flexible transformation capacity of a thermal power unit. The method comprises: based on the theoretical power model of renewable energy generation in the target year, a target annual renewable energy waste rate range is determine according to that system flexibility to adjust the capacity range, according to the range of renewable energy discarding rate in the target year, and then determine the flexibility transformation capacity of the thermal power unit according to the flexibility demand capacity of the whole power network, themethod overcomes the problem that the prior art is easy to be transformed into insufficient scale or too much capacity, Considering the relationship between the demand of flexible transformation capacity and the installed capacity of renewable energy, the calculation method mentioned above can effectively calculate the flexible transformation capacity of thermal power units, and realize the coordinated optimization of the transformation capacity and the installed capacity of renewable energy.