In the past few decades, there are five major methods that can successfully perform X-ray phase-contrast imaging, namely two-beam interferometer, crystal diffraction enhancement imaging, propagation-based imaging, grating-based X-ray interferometer and the coded aperture X-ray phase contrast imaging. Among the above five methods mentioned above, grating-based X-ray interferometer has drawn a lot of attention and made much progress due to its compatibility with X-ray source with large spot size and good image quality. In a grating-based X-ray interferometer, absorption, phase-contrast and dark-field signals can be simultaneously obtained from the same data. Generally speaking, the phase-contrast signal is created by the X-ray refraction after passing through the object, which is more advantageous than the absorption signal for soft materials. Moreover, the dark-field signal, which is regarded as small-angle-scattering information, is sensitive to the density fluctuations onmicrometre length scales. However, grating-based X-ray interferometer is limited by the small size of the absorption grating in clinic applications. It is a great challenge to fabricate the absorption grating over large area and high aspect ratio. To a certain extent, X-ray interferometer based on inverse geometry grating can remove the limitation of absorption grating on the field of view. It interchanges the position of the X-ray source and the detector and doesn′t need any analyzer grating. But its high system magnification can also reduce the field of view. Therefore, a dual phase grating interferometer is proposed to address the above difficulties. It consists of two phase gratings and can generate fringes with periods of tens to hundreds of microns. Due to the lack of absorption grating, the dual phase grating interferometer can realize X-ray phase-contrast and dark-field imaging with large field of view and high dose utilization. In the dual phase grating interferometer, the Lau condition affects the fringe visibility, which in turn affects the signal-to-noise ratio of the system. For the Lau condition of the dual phase grating interferometer, some researchers made use of the intuitive geometric relationship to calculate the transverse fringe shifts caused by the transverse shifts of the X-ray source on two phase gratings respectively, and then subtracted the two fringe shifts from each other to obtain the Lau condition. They thought that for the dual π-phase grating interferometer, the source grating period under the polychromatic illumination was twice as long as that under the monochromatic illumination. However, their theory could not explain the following questions. For the Talbot-Lau interferometer, the period of source grating is the same whether it is illuminated by the monochromatic X-rays or polychromatic X-rays. For the sensitivity of the dual phase grating interferometer, most researchers directly derived it from the sensitivity model of the Talbot-Lau interferometer. The sensitivity of the dual phase grating interferometer is valid when the system is arranged as two cascaded Talbot-Lau interferometers. However, there are still some problems to be addressed in the sensitivity of the dual phase grating interferometer, such as unreasonable physical models and incomplete theoretical results, which restrict the improvement of system sensitivity. Therefore, a new sensitivity model for the dual phase grating interferometer is proposed here: the transverse fringe shift produced by the object is equal to that produced by the position change of the X-ray source. The new sensitivity model converts the X-ray refraction by an object into the position change of the X-ray source. In addition, another key step in calculating the sensitivity of the interferometer is to use the Lau condition to connect the position change of the X-ray source with the transverse shift of the fringe. Using the new sensitivity model above, the sensitivity of the dual phase grating interferometer and Talbot-Lau interferometer are successfully obtained, which provides theoretical support for the optimization of the dual phase grating interferometer.