In order to achieve the multi-user broadcast authentication in WSNs (Wireless Sensor Networks) defending against DoS (Denial-of-Service) attack, a multi-user broadcast authentication protocol in WSNs was proposed based on the improved ECDSA (Elliptic Curve Digital Signature Algorithm) with partial message recovery. The authenticity of the user public key is verified by using bloom filter, which reduces the storage overhead of sensor nodes. The bogus messages are filtered using pre-authentication scheme based on one-way Hash key chain. Theoretical analyses show that, the proposed protocol can resist active attack, compromise attack, and DoS attack. Compared with DDA-MBAS, the proposed protocol reduces the energy consumption of sensor node by about 41.3% while improving the scalability.