A boron neutron capture therapy system, comprising a neutron capture therapy device (100), a photon emission detection device (200), and a treatment bed (300). Neutrons are irradiated by a born-containing (10B) medicine and then produce gamma rays； the photon emission detection device (200) comprises a detecting portion (201) surrounding the periphery of the treatment bed (300) and detecting the gamma rays produced by the neutrons irradiated by the born-containing (10B) medicine； the detecting portion (201) comprises a first detecting portion (202) and a second detecting portion (203)； the first detecting portion (202) and the second detecting portion (203) can be distant from or close to each other so that the detecting portion (201) can form a ring with the radius being increased or decreased； and the ring surrounds the treatment bed (300). The photon emission detection device (200) for use in the boron neutron capture therapy system can change the radius of the ring, surrounding an irradiated object, of the detecting portion (201) according to the actual condition in the boron neutron capture therapy so as to improve the detection precision of the photon emission detection device (200).
