With the rapid development of the Internet industry and the improvement of the emphasis on the information security

the CAPTCHA such as text

images and sound has been applied to every corner of our lives. Among of these CAPTCHA

text CAPTCHA are becoming most widely used for its simplicity and not relying on third-party ancillary equipment. To increase the diffi culty of identifying for the machine

the design of text CAPTCHA become more complex affecting the ease of use and causing the users dissatisfi ed.This paper reviews the development and research status of the CAPTCHA

based on the design principles of the text CAPTCHA and converting designing ideas

an implicit CAPTCHA is given in this paper ensuring the diffi culty of recognition for the machine and effectively reducing the complexity of the user login. Practice shows that this method is practical and can achieve good results in actual use.