Coherence of interferometric synthetic aperture radar (InSAR) complex image pair is a fundamental observable in interferometric radar measurements, which is usually measured by comparing the radar return across several nearby radar image pixels and has found diverse applications. This paper proposes a coherence estimation method which requires three arbitrary parts of the two complex images to implement interference. The proposed method can also be used to co-register InSAR image pair, which means that the imaginary part of the master image can be left away. In addition, an improved quality-guided phase unwrapping method is forwarded with the quality map generated by the coherence estimation method we have recommended above. A look-up table is adopted to reduce the processing time. The experimental results show that both methods are effective and greatly reduce the phase unwrapping time compared with the existing quality-guided methods.