The solubility of the ternary system of NiCl₂ -(CH₂)₆N₄?HCI-H₂O at 30℃ has been investigated by the phase-equilibrium method，the refractive indexes of the liquid phases have also been determined separately. The results show that the double salt which has the composition of 2NiC1₂ -(CH₂)₆N₄?HCl?9H₂O is formed in the cited ternary system. The thermochromic chracteristic of the equilibrium solid phase at 13℃ have been studied and its thermochromic mechanisms are inquired preliminarily.