The invention relates to a device and a method for measuring the polarization state by a two-slit interference method, which belong to the photoelectric detection technology. The device comprises a polarizing disc, a light baffle plate and a CCD detector, wherein two slits are arranged on the light baffle plate, other parts on the light baffle plate except the two slits on the light baffle plate are completely in a light-tight mode, and the polarizing disc, the light baffle plate and the CCD detector are sequentially arranged in the light path direction. The polarizing disc is covered on one slit of the light baffle plate, then, the light baffle plate and the CCD detector are sequentially fixed in the light path direction, and parallel incoming rays are irradiated on the light baffle plate and are imaged on the CCD detector through two slits on the light baffle plate for forming interference strips. The invention only uses one polarizing disc, in addition, the polarizing disc is not needed to be rotated, the device is simple, and the invention has high experimental precision. No wave length selecting device exists in the whole experiment device, and the polarization state of the incoming rays at any wave lengths can be measured.