The invention belongs to the field of radiation measurement, and particularly relates to a plastic scintillator doped with the neutron-sensitive material lithium and a method thereof for measuring thermal neutrons. According to the invention, lithium-contained raw materials are added into the plastic scintillator. The lithium-doped plastic scintillator and a photomultiplier tube are connected and are sealed inside an opaque housing so as to form a probe. The probe is placed in a measurement environment and the nuclear reaction occurs between incident neutrons and the lithium-contained raw materials in the plastic scintillator, so that the materials are in an excited state. During de-excitation, photons are sent out. The number of photons is proportional to the number of incident neutrons. Meanwhile, through recording the electrical impulses semaphore of the photomultiplier tube per unit time, the neutron fluence can be calculated, so that the measurement of the neutron fluence is realized. The neutron fluence is multiplied by a fluence-dose conversion coefficient to obtain the neutron dose equivalent. The plastic scintillator and the method are low in cost, wide in application range and high in detection efficiency.