In engineering design and calculation, flow distribution is complex and often needs a large amount of calculation, and the calculation of the system that the pipeline structure size is limited by surrounding space is more complex. With nuclear power stations planetary reducer internal flow distribution as an example, the inverse method was used to calculate the flow distribution. The calculation method is to partition the solution steps according to the constraint condition, calculate step by step to get pipeline diameter, and finally achieve the flow distribution of pipeline system. This method is able to effectively solve the flow distribution of pipeline system whose space size is limited, and can also be applied for the flow distribution calculation of common pipeline system.