Abstract: To compare communication link importance in communication networks invulnerability，a algorithm based on link-contraction is proposed.The algorithm compares the number of spanning trees after every link is contracted，and estimate every link importance in communcation networks invulnerability，the more the number of spanning trees，the more important this link，and the more its effection to communication networks invulnerability.Experimental results show the algorithm is simple and can reflect link importance in communication networks invulnerability more precisely.