The invention discloses a positioning system and a positioning method for a radioactive source. The positioning system comprises a detector, an analog-digital converter and a signal processing module,wherein the detector comprises a photomultiplier and a cylindrical scintillation crystal used for receiving radiation rays, and a detection signal is generated through the photomultiplier; the analog-digital converter is connected with the detector through a pre-amplifier; the signal processing module is used for generating inconsistency with diffusion by the light, which is vertical to a bottomsurface direction, of the cylindrical scintillation crystal; according to an amplified detection signal subjected to analog-digital conversion, a sampling waveform is generated to carry out signal screening so as to judge a distance between a radiation ray acting position and the photomultiplier; and the distance distribution situation of the radiation ray acting position and the photomultiplier is calculated to determine a radiation source incidence direction, wherein the length, which is vertical to the bottom surface direction, of the cylindrical scintillation crystal is greater than presetlength. The system has the following advantages of simple equipment and low cost, can be used for assisting in radioactive source positioning, and avoids blind search.