Synthetic Aperture Radar Interferometric（InSAR）processing depends on phase unwrapping to convert phase to topographic height.However

in rugged terrain the signal decorrelation caused by layover and shadow is one of the limitations for InSAR topographic height reconstruction

because the phase unwrapping is difficult.A new algorithm is proposed in this paper to reconstruct high resolution digital elevation model（DEM）with the help of existing coarse SRTM DEM（about 90m×90m）.Firstly

the simulated phase from coarse DEM is removed from the ERS-1/2 interferogram.Thus the residual interferogram is easier for phase unwrapping.By calculating the variance of unwrapped residual interferogram

a proper threshold is selected to extract the noise phase of layover and shadow

and then the noise area are substituted by surrounding average phase to recover the phase approximately.After converting the residual phase to height

SRTM DEM is added to reconstruct the topographic height.The result shows that this method can not only solve the problem of phase unwrapping for layover and shadow in rugged terrain

but also improve the height accuracy and topographic detail feature of coarse resolution DEM.