The characteristic parameters of the neutron radiation fields produced by an in-hospital neutron irradiatior for boron neutron capture therapy(BNCT), such as neutron energy spectra, neutron flux density and neutron absorbed dose rate and their spatial distributions in free air, etc., were measured to verify the design effect. According to the traits of the irradiators, a measuring system composed of a multi-sphere spectrometer, a 235U fission ionization chamber, gold foils, a tissue equivalent proportional counter and thermoluminescent detectors was established. In order to improve the energy resolution in epithermal energy range and the ability to discriminate different neutron components, the Monte Carlo method was used for optimizing the detectors. The preliminary results show that the neutron flux densities of the irradiators achieve the expected levels.