The thermodynamic equilibrium diagrams of CaMoO₄-CaSO₄-H₂SO₄-H₂O, CaMoO₄-HCl-H₂O and CaSO₄-CaCl₂-HCl-H₂O systems at 298 K were established. The calculation results demonstrated that HCl displays a much higher solubility of CaSO₄ than H₂SO₄. The leaching mechanism of Mo from CaMoO₄ calcine was systematically investigated from the perspective of the micro particle properties variation. HCl exhibits an excellent leaching performance for Mo from CaMoO₄ calcine due to the elimination of surface coating and the dissolution of a mass of Mo embedded in CaSO₄ matrix. Excellent Mo leaching efficiency of 99.7% was achieved under the optimal conditions of decomposing CaMoO₄ calcine by 2.4 mol/L HCl with a liquid/solid ratio of 10:1 at 50 °C for 60 min. Based on the experimental results, a highly efficient and green cycle leaching process of molybdenum from molybdenite was proposed, which eliminated surface coating and physical entraining, and converted most of the calcium from CaMoO₄ calcine into high purity gypsum by-product.