The invention belongs to the field of catalytic materials, and particularly relates to MoO3 nanocrystal with high grain boundary density which is not lower than 30,000 m/mg. In addition, the inventionfurther provides a preparation method for obtaining the MoO3 nanocrystal through oxidizing and sintering porous Mo at 450-550 DEG C. The invention also provides an application of the MoO3 nanocrystalwith high grain boundary density as a catalyst for preparing aryl formic acid by oxidizing aromatic primary alcohol. Research shows that the grain boundary density of the MoO3 nanocrystal has great influence on the catalytic oxidation performance of the aromatic primary alcohol, and further research finds that the MoO3 nanocrystal can show excellent catalytic activity in the oxidation reaction ofthe aromatic primary alcohol under the condition of controlling the high grain boundary density to a required value, and has excellent catalytic conversion performance and excellent cycling stability.