Abstract: The Feynman-α method is one method of neutron noise analyses. The α eigenvalue and the subcriticality of a subcritical system can be derived from the Feynman-α formula based on the theory that in a multiplying nuclear system, and the distribution of neutron detector counts deviates from Poisson distribution. The traditional Feynman-α formula was developed based on a one-region one-group model, which was unable to precisely describe a system with a core surrounded by the reflector. Therefore, the Feynman-α formula based on two-region model was developed, with the consideration of all types of reactions including neutron capture, neutron induced fission, neutron transition from one region to another, and neutron detection. One group of delayed neutrons was also considered. By solving this Feynman-α formula, the analytical solution was given and can be used to calculate the subcriticality of the system.