Wireless Sensor Networks (WSNs) are already employed in numerous applications including military, industry, and health. Considering the inherent limitations in WSNs, the security is a critical issue in WSNs. The declared security functions of each device should be well verified. In this paper, a security testing method based on security level is proposed for WSNs. Meanwhile, an experimental security testing platform is implemented. The test results show that the platform is a feasible platform for assessing device security levels in WSNs.