The method includes steps: partitioning data in each item of service in instant communication system (ICS) into private data and non-private data; when the said service is activated, the method calls non-private data in service; after second authentication is passed, the method calls private data in the service. The invention can make statistics for number of accessing each data of service in ICS. When accessing number exceeds prearranged number, the method puts the data as private data; otherwise, puts the data as non-private data. The invention can pick up time of generating data of servicein ICS. When generating time is after setting up time, then the data is put into non-private data; otherwise, the data are put into non-private data. The invention also discloses system of second authentication in ICS. Features are: satisfying requirement on security of ICS, and convenience and shortcut for user to use each services in ICS.