Four kinds of ternary phase diagrams of alkali bis(fluorosulfonyl)amides, LiFSA–NaFSA–KFSA, LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA and NaFSA–KFSA–CsFSA, have been constructed. All the ternary systems possess the only one eutectic point around equimolar composition with the temperature of 309–318K. The electrochemical window of the eutectic LiFSA–KFSA–CsFSA ((x Li, x K, x Cs)=(0.30, 0.35, 0.35)) is as wide as 5.4V at 323K with the cathode limit being lithium metal deposition. The electrochemical window of the eutectic NaFSA–KFSA–CsFSA ((x Na, x K, x Cs)=(0.40, 0.25, 0.35)) is 4.9V at 323K with sodium metal deposition as the cathode limit reaction.