Decorrelation of speckle patterns restricts the implementation of speckle pattern interferometry. A method to increase the speckle correlation and to suppress the decorrelation phenomena is proposed by using a cubic phase mask for the phase modulation. The relationship between the speckle correlation and speckle sizes is analyzed, and the mechanism is elaborated that the phase modulation suppresses the decorrelation of speckle patterns. The feasibility of the method of phase modulation is verified by simulation. Then the measurement system of phase modulation speckle pattern interferometry is setup. Object lights transmit through a 4f system with a cubic phase mask on the phase surface, therefore the phase modulation can be conveniently achieved. The results of contrast experiments show that the transverse correlation size and the longitudinal correlation size of modulated speckle patterns can be increased by about 1.7 and 2.5 times, respectively. The decorrelation of speckle patterns can be effectively suppressed. Moreover, the correlation area of speckle pattern interferometry, in which well-observable fringes are obtained, can be increased by about 2.5 times, and the partial contrast of fringes in speckle pattern interferometry can be enhanced.