Multi-controller is a promising control plane solution for the large-scale Software-Defined Networks (SDN). Some existing works (e.g., OpenFlow 1.2) propose to use backup controllers named slave controllers to achieve fault- tolerance in the control plane. In this paper, we identify the unreasonable slave controller assignment could cause the controller chain failure and eventually crash the entire network. We consider some important factors for designing fault-tolerant control plane and formulating Slave Controller Assignment (SCA) problem. SCA is an NP-complete problem, and we solve it with Adaptive Slave Controller Assignment (ASCA) scheme, which adaptively assigns slave controller according to load variance difference. The numerical results validate the efficiency of ASCA.