Fission chamber, based on the fission reaction of Uranium 235 and neutron, is used to detect neutron in nuclear power plant (NPP). The energy causing by fission reaction is so large that the neutron detection won’t be affected by γ. Therefore, fission chamber is widely used to measure neutron flux level of reactor in commercial pressurized water reactor, e.g. AP/CAP/HPR nuclear power units [1]. The working modes of fission chamber are pulse mode, AC mode and DC mode. These three working modes overlap each other, and should be switched according to flux level, so that the fission chamber can detect neutron flux of up to 10 orders of magnitude. It is particularly important to implement smooth switching between modes, which is favorable for the linearity and stability of output current, and it is also of great significance for the safety of nuclear power unit. Through in-depth research of signal characteristics and process mode of different working modes, a set of overlap optimization schemes for different working modes were explored with independent design rights, and has been applied to nuclear power units for the first time. The feasibility and reliability of the schemes were verified according to the onsite data during the reactor startup. In view of the fact that fission chamber technology will be widely used in the future nuclear power industry, this research has certain reference and enlightenment for commissioning, operation and maintenance of nuclear instruments in NPP.