If i install Cisco any connect inside my iPhone, can our system admin access my iPhone without my knowledge
Cisco anyconnect is nothing more than a VPN app that is designed to connect back to your work network securely.

They likely have resources on the Intranet that are not available to the outside world and the VPN will put you onto that network using that Cisco VPN software.

Generally speaking this will not give network administrators (your IT dept.) access to your phone. VPN software is designed to give you access to their systems. So unless your IT people have inserted a trojan into Cisco's VPN software (doubtful in the extreme) you are safe from your IT department gaining access to your personal device using that software.