Continuous phase plate(CPP) is critical diffractive optical element used for beam shaping in high-power laser system. But the large-aperture interferometer is unsuitable for testing it for its spatial period with several millimeters and the large phase gradient. A sub-aperture stitching method by small-aperture and high lateral-resolution interferometer is proposed to realize wavefront testing of CPP. A novel interpolation algorithm is developed for circular aperture and the equipment is set up. It tests a CPP with size of 330 mm×330 mm.The experimental result shows that the maximum testable slope of this CPP reaches 3 μm/cm with millimeter period,while the testing repeatability is 6 nm(root mean square). The simulated spot diagram result of far field based on tested wavefront is in good agreement with tested result in high-power laser system.