In the digital measurement for free-form surface,Image mosacis are needed to achieve the whole measurement for the smaller measuring field of sensor unit or the feature limitations of the measure object.Viscous target can solve the transformation and the integration of several coordinate systems in space image mosaics.In this paper,an identification technology for the planar multi-arc viscous target was presented which is based on the invariable curve-moment.This method can identify viscous target exactly and has strong robustness.The fractional error of 3-D space distance measured with this method excels 0.6%.