In this paper, the YBa₂Cu₃O_7-x (YBCO) films were prepared on LaAlO₃ (100) substrate using a low-fluorine sol-gel method. The effect of the oxygen content during firing YBCO films at 725 °C on the critical current density (J_c) of the resulting YBCO films were investigated. It is found that all the YBCO films fired in the oxygen content scale of 100~1700 ppm exhibit good biaxial-texture characteristics. However, the YBCO film is less dense when the oxygen content is low and thus the critical current density (J_c) of the film is inferior. The film surface becomes denser with increasing the oxygen content during the firing process, and J_c of the film reaches 4.3 MA/cm₂ when the oxygen content is elevated to 300 ppm. Furthermore, many copper-rich second phases appear on the films surfaces and become more gradually with the further elevating the oxygen content, resulting in the decline of J_c.