In the wireless sensor networks (WSNs), users sometimes need to access real-time information from a certain sensor node. In order to prevent unauthorized users from acquiring information from the sensor node, a lot of authentication schemes suited for the WSNs condition have been proposed. Recently, Das has proposed a secure and light-weight authentication scheme for the WSNs based on three factors, claiming that it can withstand various attacks. But after a careful analysis, there are still several security problems as follows: a) The scheme has some design flaws; b)The scheme may suffer sensor node impersonation attack; c) The scheme may suffer repeat registered attack; d) When the user access the sensor node, the scheme cannot protect the identity information of the sensor node. To solve these problems, a new scheme using the secure sketch algorithm is presented in this paper. The security of improved scheme has been analyzed by ProVerif. Thorough analysis shows that the presented scheme can provide stronger security and slight lower computation at client than Das’s protocol. What’s more, it can overcome repeat registered attack and achieve sensor node’s identity anonymity protection.