An X-ray detecting system based on square polycapillary X-ray lens was designed, which had a very small X-ray collection angle. The square polycapillary X-ray lens is an X-ray control device based on total X-ray reflection. The X-ray in large range can be focused on an X-ray CCD detector by the square polycapillary X-ray lens. By measuring the transmission characteristic of the square polycapillary X-ray lens and creating the data modelling, the data detected by the X-ray CCD detector could be corrected and the information of the X-ray in the entry end of the square polycapillary X-ray lens could be restored. The transmission characteristic of the square polycapillary X-ray lens was simulated by the light trajectory tracking method. The results show that this system is suitable for large area X-ray imaging when the energy of the X-ray is lower than 20 keV. It is also suitable for collecting X-ray to improve the detection efficiency when the energy of the X-ray is lower than 14.6 keV. The above characteristics show that this system is not only suitable for some special detecting such as pulsar navigation, but also suitable for normal X-ray detecting.