A new method of analysis of intensity modulation is proposed for phase unwrapping of equal-step phase-shifting interferometry. By analyzing the intensity modulation of interference speckle patterns, a variety of noise, inconsistence and invalid data area can be identified. A binary and decimal weight can be constructed by this method for weighted phase unwrapping that utilizes least-squares approach and enhances the immunity of phase unwrapping from kinds of disturbing factor. The application of this method to Carré algorithm and Stoilov algorithm is discussed. The experimental result shows the validity and praticality of this method. A comparison of the performance of binary and decimal weight in weighted least-squares phase unwrapping is provided.