The present invention provides a sample tube for polymerase chain reaction. The sample tube includes a sample tube body having an annular side wall and a bottom wall connected to annular side wall, wherein the annular side wall and the bottom wall form an accommodating space for accommodating a reaction mixture; and a first extension member extending from the bottom wall to the accommodating space, wherein, to carry out PCR, the reaction mixture immerses the first extension member, such that the light from the light source enters into the reaction mixture via the bottom and the bottom wall of the sample tube and the first extension member sequentially.