This disclosure relates to an X-ray reflectometry apparatus and a method for measuring a three-dimensional nanostructure on a flat substrate. The X-ray reflectometry apparatus comprises an X-ray source, an X-ray reflector, a 2-dimensional X-ray detector, and a two-axis moving device. The X-ray source is for emitting X-ray. The X-ray reflector is configured for reflecting the X-ray onto a sample surface. The 2-dimensional X-ray detector is configured to collect a reflecting X-ray signal from the sample surface. The two-axis moving device is configured to control two-axis directions of the 2-dimensional X-ray detector to move on at least one of x-axis and z-axis with a formula concerning an incident angle of the X-ray with respect to the sample surface for collecting the reflecting X-ray signal.