The invention discloses a preparation method and an application of dopamine-modified molybdenum oxide quantum dots. The preparation method comprises the steps of: firstly, modifying molybdenum oxide quantum dots by using dopamine, so that the surfaces of the molybdenum oxide quantum dots have more oxygen and nitrogen active sites, then using the dopamine-modified molybdenum oxide quantum dots as anovel uranium (VI) indicator, performing qualitative and quantitative analysis on uranium (VI) through the fluorescence intensity of the dopamine-modified molybdenum oxide quantum dots before and after the uranium (VI) is added, and adjusting an optimal test concentration and test environment. In combination with a marking method, the trace uranium (VI) in the environment can be accurately detected, and the practical application value is relatively high.