Abstract: DPFA new type of differential interferometer using a defocused 4f imaging system is described. The interferometer is stable and easy to adjust with a ring structure. The shear amount and fringe spacing can be adjusted independently with the defocused 4f design. High spatial resolution and readable fringes can be obtained simultaneously in dense plasma diagnosis. As an example, the DPF plasma was diagnosed by the 4f system. Interferograms were acquired at different time during pinch process. The radial distributions of local electron density were calculated. The maximum local electron density above the anode end was measured about 1.2×1019 cm-3 and that of the outer plasma shell 2×1018 cm-3 when plasma began pinching.