A novel method based on machine vision is proposed for measuring the effective d31 coefficient. Based on the best estimation of gray scale of interference pattern, the sensitivity of the twin-beam interferometry is significantly improved in comparison with previously reported system. Firstly, the interference pattern grabbed by machine vision system is extracted using the algorithm developed in this work. Secondly, using gray-scale pattern with 256 gray-levels the measurement accuracy of the interferometric optical path difference of the interferometer can reach 1/1024 of the wavelength of the laser theoretically. So the experimental results of a PZT pipe in the method are greatly accurate.