The judgment policy and quality of investors' decision directly relate to the efficiency and development of security market. Based on lens model, this paper designed an experiment to explore the judgment policy and quality for professional and nonprofessional investors and to compare their difference. According to the response from 99 mutual fund managers as well as researchers, and 258 employees of security companies, the main results of this study are as follows: (1) Both the professional and nonprofessional investors adopted linear judgment policy, especially for the professional investors. In addition, the professional investors used structured checklist strategy to treat information, that is, they focused on little information only. On the other hand, the nonprofessional investors used balanced strategy. (2) All of the judgment quality indicators (i.e., stability, consensus, and self-insight) consistently showed that professional investors have better performance than nonprofessional investors. (3) Experience variable is significantly for professional investors, but not for nonprofessional investors. It seems that only professional investors can learn form experience.