The invention relates to a method and apparatus for space-distributed monitoring of a ionising radiation field. The claimed method is based on the utilization of an optic fiber long-period sensor grating and consists in measuring in real time the central wavelength modifications for each sensor, in case of the exposure to a ionising radiation field, the wavelength modification being proportional with the total dose to which the grating was exposed, and, by arranging the optic fibre sensors, forming the sensor grating, in certain positions, the spatial variation, 3D, of the dose can be monitored in various points of interest of the ionising radiation field. The claimed apparatus comprises an optic fiber long-period sensor grating (15), coupled by means of some connection optic fibers (16), an optic fiber circulator (19) and an optic fiber multiplexer (22), to an optical frequency domain reflectometer (25) and an optic fiber Bragg sensor grating (27) coupled, by means of other connection optic fibers (28) and of the multiplexer (22), to an optic fiber interrogator (32), both the optic reflectometer (25) and optic fiber interrogator (32) being connected to a central unit (33) which measures central wavelength values of optic fiber long-period sensors (15) and calculates the total dose and the dose rate to which the sensors (15) were exposed, the results being corrected with the temperature variations determined by the central unit (33) by measuring the central wavelength variations of the optic fiber Bragg sensor grating (27).