In order to reduce the vibration error of phase-shifting interferometry (PSI) and improve the demodulation accuracy of the wavefront phase, a demodulation method based on a component synthesis and gradient projection for PSI is proposed. For two sequences of interference images with a phase shift of 0, π/2, the method first corrects the vibration error by the pre-processing mechanism and multi-component synthesis. At the same time, two frames of interference images with a phase shift of 0, π/2 are synthesized. Then the gradient projection algorithm is used to filter the background light of the synthesized image, and the wavefront phase is demodulated based on the arctangent operation. The experimental results show that the percentage of pixels demodulated effectively at the truncated interface is more than 90%. Compared with the traditional demodulation method, the number of pixels demodulated effectively (demodulation accuracy) is improved.