Conventional trifluoroacetic acid metal organic chemical solution deposition (TFA-MOD) method for the preparation of YBa₂Cu₃O_7-δ (YBCO) superconducting layers follows the BaF₂ growth mechanism. In this work, a novel chemical solution method based on BaCl₂/BaF₂ pathway for the growth of YBCO superconducting thin films was carried out. The effects of Cl addition on grain orientation, microstructure and superconductivity of YBCO thin films were investigated, and mechanism of the phase transition of YBCO thin films in the BaCl₂ pathway was explored by thermochemical calculations of the growth reaction. The results show that the Cl addition inhibits a-axis grain orientation and promotes nucleation of c-axis grains. Onset transition temperature (T_c-onset) of the YBCO 2-layer film is about 89.6 K without significant change after Cl addition. But its critical current density (J_c) is significantly increased to 2.07 MA/cm² (77 K, self-field). Meanwhile, the phase transition of the growth reaction process shows that Cl is preferentially combined with Ba to form BaCl₂ as much as possible, effectively avoiding the formation of BaCO₃. All results of this study indicate that Cl addition facilitates preparation of YBCO superconducting thick films, and provides a new idea for the preparation of YBCO by MOD method.