The invention particularly provides a downhole ray type water content monitor which comprises a shell. The upper end and the lower end of the shell are provided with an upper connector and a lower connector respectively. A central passageway vertically penetrates through the shell. A radioactive source installation base, a radioactive source, a transmission detector, a scattering detector, a circuit board and a battery pack are arranged between the shell and the central passageway. The radioactive source is installed on the radioactive source installation base obliquely. The transmission detector is arranged on a light ray circuit of the radioactive source and located on the side opposite to the radioactive source. The scattering detector is arranged above the radioactive source, and the included angle between the scattering detector and the light ray circuit of the radioactive source is 90 degrees. The battery pack is installed at the lower end of the shell. The circuit board is arranged between the scattering detector and the battery pack. The radioactive source and the circuit board are electrically connected with the battery pack. By means of the downhole ray type water content monitor, errors which are caused by contained water to water content measurement are eliminated, the water contents of all layer sections of a tubular column multi-section fracturing horizontal well are monitored and rapidly measured, the water content measuring cycle is shortened, and the water content measuring accuracy is improved.