Microgrid transition between standalone and grid-connected modes is a promising alternative to provide the grid with increasing flexibility and availability. However, transition smoothness relies heavily on control topologies and corresponding parameters, which thus remains challengeable. Existing microgrid transition strategies have two major deficiencies: 1) Inverter control mode alters subjected to microgrid operating mode, for instance, the inverter in current control will switch to voltage control when microgrid disconnects to the utility grid; 2) Controller parameters are selected based on practice and experience, where a systematic and efficient approach does not exist. Motivated by these limitations, in this paper, an autonomous control strategy is proposed for microgrid smooth state transitions. It is highlighted in the following aspects: 1) The cascaded control strategy enables smooth state transition within a single control structure, which permits controller independent of mode switching; 2) Nonlinear-Simplex based algorithm is interfaced with electromagnetic transient simulations, searching for optimal controller parameters in order to minimize voltage deviation in a chain of microgrid events. The effectiveness of the control framework is validated with simulations in PSCAD/EMTDC and RTDS.