In the conventional 4f phase coherent imaging measurement technology, phase objects were used to convert the phase change generated by the measured material into a directly observable amplitude change to achieve the measurement of the nonlinear refractive index of the material. However, the thickness of the phase object was fixed, and the phase delay changes with the laser wavelength, and a suitable phase object needs to be replaced in the measurement. The influence of different laser wavelength on the phase shift of the phase object was theoretically analyzed, and the influence of the radius of the phase object and the phase shift on the sensitivity of the 4f phase coherent imaging system was discussed in detail. A phase-adjustable phase object was realized by using the phase delay of two beams in Michelson interferometer instead of the conventional phase object. Because the phase delay of the two beams of Michelson interferometer was continuously adjustable, it makes the sensitivity of measurement at different shapes and different wavelengths of laser optimal. This method further improves the 4f phase coherent imaging measurement technology, not only solves the deficiencies of the conventional phase object, but also improves the accuracy of the measurement.