The axial decorrelation of speckle patterns is one of the key problems which restrict the use of speckle interferometry. A method of speckle field phase encoding modulation is proposed to increase the speckle longitudinal correlation so that the contrast of the speckle interference fringes can be improved, and then the measurement range of the object surface deformation can be extended. The wavefront coding technique is used to code the speckle fields and a cubic phase mask is set at the position of aperture. The feasibility of this method in increasing the longitudinal correlation of speckle fields is analyzed firstly. The effect that the cubic phase mask in the 4f system can effectively increase the longitudinal correlation of coded speckle field is validated by simulation and experiment respectively. The results show that the longitudinal speckle size increases from 0.17 mm to 0.4 mm and increases by about 2.5 times.