YBa_2Cu_3O_(7-δ)(YBCO) films were deposited on(100)-oriented LaAlO_3(LAO) single crystal substrates by the dip-coating process using low-fluorine solution.Their microstructures were characterized with the aid of X-ray diffractometry,scanning electron microscopy and high-resolution transmission electron microscopy.Their superconducting properties were measured by the standard four-probe method.The experiment results show that the film obtained under high enough humidity conditions exhibits better c-axis texture and superconducting properties than the film under a relatively low humidity conditions.Based on the classical nucleation and chemical reaction thermodynamics theory, the underlying crystalline and growth mechanisms of YBCO films under certain humidity conditions are explained in combination with our experimental results.It is suggested that the unreacted intermediate phases such as BaF_2 and CuO aggregated in the YBCO grain boundary will cause lattice distortion in the YBCO matrix and further induce the formation of a-axis oriented YBCO grains as crystallization proceeds.Therefore,it is believed that the relative content of water vapor within the heat-treatment atmosphere plays quite an important role in the preparation of c-axis oriented YBCO film with good superconducting properties. YBa_2Cu_3O_ (7-δ) (YBCO) films were deposited on (100) -oriented LaAlO_3 (LAO) single crystal substrates by the dip-coating process using low-fluorine solution. Theseir microstructures were characterized with the aid of X-ray diffractometry, scanning electron microscopy and high-resolution transmission electron microscopy. their superconducting properties were measured by the standard four-probe method. experiment of show that the film obtained under high enough humidity conditions exhibits better c-axis texture and superconducting properties than the film under a relatively low humidity conditions. Based on the classical nucleation and chemical reaction thermodynamics theory, the underlying crystalline and growth mechanisms of YBCO films under certain humidity conditions are explained in combination with our experimental results. It is suggested that the unreacted intermediate phases such as BaF_2 and CuO aggregated in the YBCO grain boundary will cause lattice distortion in the YBCO mat rix and further induce the formation of a-axis oriented YBCO grains as crystallization proceeds.Therefore, it is believed that the relative content of water vapor within the heat-treatment atmosphere plays quite an important role in the preparation of c-axis oriented YBCO film with good superconducting properties.