Due to the open nature of wireless networks, power wireless private networks (PWPNs), as the carrier of smart grid service, must provide highly reliable and secure communication capabilities. The security risks of terminals, wireless channels, base stations, core networks, host systems and data applications are analyzed. Aiming at the multi-service characteristics of smart grid, a security architecture based on service isolation is proposed in PWPNs. The architecture implements end-to-end service security isolation and can effectively combat different security threats from outside.