A subcritical multiplying assembly (SMA) was employed to improve the relatively low neutron fluxes of a 252Cf source, and the feasibility of using it as the neutron source for boron neutron capture therapy was explored. The Monte Carlo code MCNP was used to evaluate the effective multiplication factor keff of the entire system, the intensities and percentages of the epithermal neutron flux at the patient-end surface of the beam, and dosimetric properties of the beam in the elliptical brain phantom. The neutron beam with the SMA provides an epithermal neutron flux ~13.2 times higher than the beam without the SMA. After some optimization procedures, the beam in the final design provides a maximum advantage depth (AD) of 8.9 cm, a minimum AD of 7.3 cm, an advantage ratio of 5.5, and a therapeutic relative biological effectiveness dose rate of 4.23 cGy/min per 100 mg of 252Cf at a depth of 7.0 cm in the brain phantom. This dose rate is ~10 times higher than that provided by the beam designed without the SMA. Therefore, it is expected that the neutron beam can be more effective for treatment of tumors due to the increased therapeutic dose rates.