The Unbalanced Oil and Vinegar scheme (UOV) is a signature scheme based on multivariate quadratic equations. It has o oil variables and v vinegar variables. UOV has m equations and n variables, where m = o and n = v + o. In this paper, we define the weak key of UOV and study how to find the weak key from the public key. Second, we study the security when m > o. And our result shows that the security strengths of the current version of TTS, TRMS, Rainbow and MFE are 259 ~267.6 3DES operations.