Abstract: β-delayed neutron emission is a very important decay mode for neutron-rich unstable nuclei. The branching ratio of delayed neutron in β decay, i.e. the β-delayed neutron emission probability (P_n) plays an important role in nuclear structure, nuclear astrophysics and nuclear energy applications. In the operation of a nuclear reactor, the β-delayed neutron is the crucial factor enabling a chain reaction of nuclear fuel to be controllable. Precise P_n values thus have significant impacts on the safety control of nuclear reactors, the reactor decay heat release, and nuclear waste management as well. P_n is also an indispensable input for network calculation of the rapid-neutron process (r-process) in nuclear astrophysics, which influences on the route of r-process and the final abundance distribution. With the rapid advance of large-scale domestic and international radioactive ion beam facilities toward to very neutron-rich isotopes far from the stability line, experimental measurement of their P_n values is becoming a highlighted topic in nuclear research frontier. This paper briefly introduces the current research status of β-delayed neutron emission probability and the measurement methods. Based on the existing international ³He neutron detector arrays, a design of our own Long Helium-3 Neutron Array (LHENA) is performed to conduct P_n experimental research at domestic radioactive isotope facilities.