The formation mechanism for the equilateral triangle structure of Li₃ cluster is proposed. The curve of the total energy versus the interatomic distance for this structure has been calculated by using the method of Gou's Modified Arrangement Channel Quantum Mechanics. The result shows that the curve has a minimal energy of -22.33860 a.u. at R=5.82a₀. The total energy of Li₃ when R approaches ∞ has the value of -22.28409 a.u. This is also the total energy of three lithium atoms dissociated from Li₃. The difference value of 0.054508 a.u. for the above two energy values is the dissociation energy of Li₃ cluster, which is also its binding energy. Therefore the binding energy per lithium atom for Li₃ is 0.018169 a.u.=0.494 eV, which is greater than the binding energy of 0.453 eV per atom for Li₂ calculated in a previous work. This means that the Li₃ cluster may be formed in the equilateral triangle structure of side length R=5.82a₀ stably with a stronger binding from the symmetrical interaction among the three lithium atoms.