Abstract: Captcha is a program which can be used to distinguish between computer and human, the image verification code is a typical application of Captcha. The development of the Captcha is summarized, and the feature and design ideas of the common Captcha are also compared. Then this paper presents the attack method on common Captcha and designs the system of feature extraction of Captcha, reducing covering algorithm based on conditional information entropy is designed and applied to attack Captcha. The experimental results prove that the model achieve excellent robust and high success recognition rate.