Electrical Speckle Shearing Phase-shifting Pattern Interferometry (ESSPPI) is a kind of full-field nondestructive measurement technology and has many merits such as noncontact, full-field measurement, high precision and anti-vibration-noise, etc. ESSPPI has been used in the fields of deformation measurement and nondestructive testing. The theory and methods are studied with statistical optics in the paper and the corresponding mathematical model, which is used to describe the relation between vertical displacement gradient to object surface and distribution of speckle stripes, is set up. The algorithms of three-step method, four-step method and five-step method are deduced on the basis of above model. The factors that may reduce the phase precision are analyzed. The results of analysis indicate that the measurement precision is mainly affected by phase-shifting linear error. The phase error with five-step method is obviously less than that with three-step method or four-step method, but it can’t be eliminated. Therefore a new four-step method no affected by phase-shifting error is put forward. The difference between phase distribution gained with above method and the real phase distribution is regarded as a fixed value and can be eliminated. Four phase-shifting interference pattern figures are accessed by an experiment of aluminum plate deformation and a packaged phase distribution figure is obtained with the new four-step method. The real phase distribution figure, obtained with unpackaged algorithm, is shown in the paper to prove the validity of the new four-step method.