The Completely Automated Public Turing Test to Tell Computers and Humans Apart (CAPTCHA) identification is designed to distinguish between computers and humans and it prevents the web application programs from malicious attacks, so it has been applied widely. However, great challenges must be faced with the development of CAPTCHA identification. In order to improve the safety of the professional system, the CAPTCHA online identification based on the knowledge base, which has high security and bases on semantic questions and the professionalization of professional system, is put forward combining with the recessive CAPTCHA. The specific implementation course of the new online identification method is worked out according to the example of animal identification. The application of the verification code is suitable for people who have the corresponding professional knowledge. Because the computer has great difficulty to answer semantic information questions, which are also professional issues, so the new online identification method based on the verification of knowledge has very high security.