Abstract: The in-hospital neutron irradiator (IHNI-Ⅰ) for boron neutron capture therapy (BNCT) was constructed by Beijing Capture Technology Corporation. To obtain the neutron spectra of free beams in air, an improved active multi-sphere spectrometer was established and the relevant experimental method was also developed. The spectrometer consisted of fourteen detection units and a spherical 3He proportional counter was used as the central detector. In order to improve the resolution of the spectrometer in epithermal region, four detection units covered with different thickness boron shells were added into the routine multi-sphere spectrometer. The response functions of the spectrometer were calculated with the MCNP code, and calibrated and verified with standard ²⁵²Cf and ²⁴¹Am-Be neutron sources. The measurements were performed at 110 cm distance from exit of the irradiator, and then the neutron spectra at the exit were deduced by anti-iteration technique. The results show that there is a little discrepancy between the measured spectra and simulated ones. Therefore, the neutron spectra and the depth curve of neutron flux density were also measured by a ROSPEC spectrometer and gold foils. The results verified the reliability of the neutron spectra measured by the multi-sphere spectrometer.