Oxygen vacancies of metal oxides play a critical role in photocatalytic activity. In this work, a novel method involving the adjustment of pH and use of a surfactant is used to synthesize Sr2FeMoO6 having more oxygen vacancies. The synthesized photocatalyst (Sr2FeMoO6) is used to study the relationship between oxygen vacancies and photodegradation of methyl blue (MB) solution. Sr2FeMoO6 samples were prepared using sol-gel (S1 sample), hydrothermal (S2 sample), and thermal decomposition of a metal-organic salt using organic solvent methods (S3 sample). The photocatalytic activities of synthesized samples fell in the descending order of: S3 > S1 > S2. Similarly, the surface oxygen vacancies of synthesized photocatalysts (S1, S2 and S3) were found to be in the descending order of: S3 > S1 > S2. Based upon the comparison and analysis of S1 and S4, the effect of pH value under similar synthesis conditions was studied. Furthermore, EDTA (surfactant) was used to synthesize Sr2FeMoO6 (herein, referred to as S5) having the same pH as that of S3. The comparison of S4 and S5 samples was used to probe the effect of Sr2FeMoO6 photocatalyst, synthesized using a surfactant-assisted route. Excellent photocatalytic activity was found for sample S5, where the pH of 8 was the optimum for sol-gel preparation method. The MB solution can be degraded completely (100%) in 35 min for S5 sample. The results indicate that the lattice oxygen vacancies can promote the harvesting of visible light and narrow the band gap, while the surface oxygen vacancies can increase the separation efficiency of charge-carriers. Beside their critical role in the evolution of photocatalytic oxygen, the surface oxygen vacancies also proved helpful in separating electrons and holes.