Abstract: A large ion collider experiment (ALICE) at the large hadron collider (LHC) will be indicated to the detection of quark-gluon plasma (QGP) formed in heavy-ion collisions at TeV, and the exploration of the early universe as it was in the first few microseconds after its birth. A high precision electromagnetic calorimeter—ALICE photon spectrometer (PHOS), designed and produced by ALICE collaboration was described in the paper. It was used to detect the prompt- and decay-photons and diagnose the electromagnetic signal of quark-gluon plasma in lead-lead collisions at s_NN^1/2= 5.5 TeV. The beam test results show that PHOS is capable to detect photons efficiently up to 0.5-100 GeV/c in ALICE.