WSNs is usually deployed in opening wireless environment, its data is easy to be intercepted by attackers. It is necessary to adopt some encryption measurements to protect data of WSNs. But the battery capacity, CPU performance and RAM capacity of WSNs sensors are all limited, the complex encryption algorithm is not fitted for them. The paper proposed a light-level symmetrical encryption algorithm: LWSEA, which adopt minor encryption rounds, shorter data packet and simplified scrambling function. So the calculation cost of LWSEA is very low. We also adopt longer-bit Key and circular interpolation method to produce Child-Key, which raised the security of LWSEA. The experiments demonstrate that the LWSEA possess better “avalanche effect” and data confusion degree, furthermore, its calculation speed is far faster than DES, but its resource cost is very low. Those excellent performances make LWSEA is much suited for resource-restrained WSNs.